linux cache routines for Write-back cache policy on MIPS24KE - Kernel

This is a discussion on linux cache routines for Write-back cache policy on MIPS24KE - Kernel ; Hi, I have ported Linux-2.6.18 kernel on MIPS24KE processor. I am using write back cache policy. Could you please guide me under what cases the below cache API's are being used: - dma_cache_wback_inv() : Could you explain what exactly this ...

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Thread: linux cache routines for Write-back cache policy on MIPS24KE

  1. linux cache routines for Write-back cache policy on MIPS24KE

    Hi,

    I have ported Linux-2.6.18 kernel on MIPS24KE
    processor. I am using write back cache policy.

    Could you please guide me under what cases the below
    cache API's are being used:
    - dma_cache_wback_inv() : Could you explain what
    exactly this function does
    - dma_cache_wback() : This function write back the
    cache data to memory
    - dma_cache_inv : This function invalidate the cache
    tags. so subsequent access will fetch from memory.

    Once I looked the above function definitions in
    linux-2.6.18/arch/mips/mm/c-r4k.c.
    All these function's implemetation are same except
    bc_wbak_inv() is called in both dma_cache_wback-inv()
    and dma_cache_wback(), where as bc_inv() is called in
    case of dma_cache_inv.

    Also, bc_inv()/bc_wbak_inv are define as null
    implementation for R4000.
    That means all three functions are doing same
    functionality in case of R4000.

    What are the difference between these three functions.
    Under what cases these functions are used.

    Please guide me if you have any links which will
    explain these API's.
    Thanks in advance.

    Regards,
    Veerasena.


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  2. Re: linux cache routines for Write-back cache policy on MIPS24KE

    On Mon, 1 Oct 2007, veerasena reddy wrote:
    > I have ported Linux-2.6.18 kernel on MIPS24KE
    > processor. I am using write back cache policy.
    >
    > Could you please guide me under what cases the below
    > cache API's are being used:
    > - dma_cache_wback_inv() : Could you explain what
    > exactly this function does


    It does both write back and invalidate.

    > - dma_cache_wback() : This function write back the
    > cache data to memory
    > - dma_cache_inv : This function invalidate the cache
    > tags. so subsequent access will fetch from memory.


    Note that 2.6.18 is old. The above functions are intended to be removed.

    Gr{oetje,eeting}s,

    Geert

    --
    Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

    In personal conversations with technical people, I call myself a hacker. But
    when I'm talking to journalists I just say "programmer" or something like that.
    -- Linus Torvalds
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  3. Re: linux cache routines for Write-back cache policy on MIPS24KE

    On Mon, Oct 01, 2007 at 10:04:32AM +0100, veerasena reddy wrote:

    > I have ported Linux-2.6.18 kernel on MIPS24KE
    > processor. I am using write back cache policy.
    >
    > Could you please guide me under what cases the below
    > cache API's are being used:
    > - dma_cache_wback_inv() : Could you explain what
    > exactly this function does
    > - dma_cache_wback() : This function write back the
    > cache data to memory
    > - dma_cache_inv : This function invalidate the cache
    > tags. so subsequent access will fetch from memory.
    >
    > Once I looked the above function definitions in
    > linux-2.6.18/arch/mips/mm/c-r4k.c.
    > All these function's implemetation are same except
    > bc_wbak_inv() is called in both dma_cache_wback-inv()
    > and dma_cache_wback(), where as bc_inv() is called in
    > case of dma_cache_inv.
    >
    > Also, bc_inv()/bc_wbak_inv are define as null
    > implementation for R4000.
    > That means all three functions are doing same
    > functionality in case of R4000.
    >
    > What are the difference between these three functions.
    > Under what cases these functions are used.


    An internal only interface to be used with I/O cache coherency.

    > Please guide me if you have any links which will
    > explain these API's.


    Easy answer, don't use them, for 2.6.24 I've queued a patch to kill this
    API. Documentation/DMA-API.txt documents how to properly deal with I/O
    coherency in Linux.

    Ralf
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  4. Re: linux cache routines for Write-back cache policy on MIPS24KE

    Hi Ralf,

    Thanks for the reply.

    Is there any problem if we use the below API's in
    linxu-2.6.18
    - dma_cache_wback_inv()
    - dma_cache_wback()
    - dma_cache_inv()

    functionality wise, especially in r4k.c i dont see any
    difference between the implementation of these APIs.

    Can we apply the 2.6.24 patch to kill these APIs on
    2.6.18 kernel? In this case what APIs i can use for
    writeback, invalidation or both?

    I couldn't find any info. related to the above API in
    DMA-API.txt. Could you please give some pointers on
    the usage/working of these APIs.

    Regards,
    Veerasena.

    --- Ralf Baechle wrote:

    > On Mon, Oct 01, 2007 at 10:04:32AM +0100, veerasena
    > reddy wrote:
    >
    > > I have ported Linux-2.6.18 kernel on MIPS24KE
    > > processor. I am using write back cache policy.
    > >
    > > Could you please guide me under what cases the

    > below
    > > cache API's are being used:
    > > - dma_cache_wback_inv() : Could you explain what
    > > exactly this function does
    > > - dma_cache_wback() : This function write back the
    > > cache data to memory
    > > - dma_cache_inv : This function invalidate the

    > cache
    > > tags. so subsequent access will fetch from memory.
    > >
    > > Once I looked the above function definitions in
    > > linux-2.6.18/arch/mips/mm/c-r4k.c.
    > > All these function's implemetation are same except
    > > bc_wbak_inv() is called in both

    > dma_cache_wback-inv()
    > > and dma_cache_wback(), where as bc_inv() is called

    > in
    > > case of dma_cache_inv.
    > >
    > > Also, bc_inv()/bc_wbak_inv are define as null
    > > implementation for R4000.
    > > That means all three functions are doing same
    > > functionality in case of R4000.
    > >
    > > What are the difference between these three

    > functions.
    > > Under what cases these functions are used.

    >
    > An internal only interface to be used with I/O cache
    > coherency.
    >
    > > Please guide me if you have any links which will
    > > explain these API's.

    >
    > Easy answer, don't use them, for 2.6.24 I've queued
    > a patch to kill this
    > API. Documentation/DMA-API.txt documents how to
    > properly deal with I/O
    > coherency in Linux.
    >
    > Ralf
    >




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  5. Re: linux cache routines for Write-back cache policy on MIPS24KE

    Hi Geert,

    Thanks for your repsonse.

    In linux-2.6.18 (for MIPS24KE processor):
    suppose if i want to do flush only then which API i
    should use?
    Similarly, if i want to do invalidation only which API
    i should use?

    Thanks again.

    Regards,
    Veerasena.
    --- Geert Uytterhoeven wrote:

    > On Mon, 1 Oct 2007, veerasena reddy wrote:
    > > I have ported Linux-2.6.18 kernel on MIPS24KE
    > > processor. I am using write back cache policy.
    > >
    > > Could you please guide me under what cases the

    > below
    > > cache API's are being used:
    > > - dma_cache_wback_inv() : Could you explain what
    > > exactly this function does

    >
    > It does both write back and invalidate.
    >
    > > - dma_cache_wback() : This function write back the
    > > cache data to memory
    > > - dma_cache_inv : This function invalidate the

    > cache
    > > tags. so subsequent access will fetch from memory.

    >
    > Note that 2.6.18 is old. The above functions are
    > intended to be removed.
    >
    > Gr{oetje,eeting}s,
    >
    > Geert
    >
    > --
    > Geert Uytterhoeven -- There's lots of Linux beyond
    > ia32 -- geert@linux-m68k.org
    >
    > In personal conversations with technical people, I
    > call myself a hacker. But
    > when I'm talking to journalists I just say
    > "programmer" or something like that.
    > -- Linus Torvalds
    >




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  6. Re: linux cache routines for Write-back cache policy on MIPS24KE

    On Mon, Oct 01, 2007 at 01:10:59PM +0100, veerasena reddy wrote:

    > Is there any problem if we use the below API's in
    > linxu-2.6.18
    > - dma_cache_wback_inv()
    > - dma_cache_wback()
    > - dma_cache_inv()
    >
    > functionality wise, especially in r4k.c i dont see any
    > difference between the implementation of these APIs.
    >
    > Can we apply the 2.6.24 patch to kill these APIs on
    > 2.6.18 kernel? In this case what APIs i can use for
    > writeback, invalidation or both?
    >
    > I couldn't find any info. related to the above API in
    > DMA-API.txt. Could you please give some pointers on
    > the usage/working of these APIs.


    dma_cache_* were never documented. They respresent the earliest attempt
    at coming up with an API that enables portable drivers and it has a few
    shortcomings and like so many early things it was never formally documented,
    so don't expect any well defined semantics. The functions never got
    formally retired probably because it somehow managed to stay under the
    radar.

    Any drivers should use the APIs documented in Documentation/DMA-API.txt
    only. The almost equivalent operation for dma_cache_* would be
    dma_sync_single and dma_sync_sg.

    Don't even dream about using dma_cache_* for anything but DMA coherency.
    They're all internal low level APIs which know nothing about Linux's
    virtual memory system.

    Anyway, it would be much easier to help you if we knew what you are trying
    to achieve with these functions.

    Ralf
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  7. Re: linux cache routines for Write-back cache policy on MIPS24KE

    Ralf,

    thanks for the detailed information.
    > Anyway, it would be much easier to help you if we
    > knew what you are trying
    > to achieve with these functions.


    Basically our target has a MIPS24KE host processor on
    which Linux runs and a networking processor (NP) which
    sits between the EMAC contoller and the host processor
    to receive/transmit the data/packets.

    This peripheral networking processor uses the physical
    addresses only. We are using write-back caching policy
    on MIPS24KE. So,
    1. when we want to transmit the packet from the host
    to peripheral processor, we need to convert packet
    buffer into physical address (CPHYSADDR) and put it
    into the NP's Tx queue, which will be sent to EMAC.
    Before converting into physical address we need to
    flush the corresponding cache entries.
    Which API should be used to achieve the above
    functionlaity in Linux-2.6.18 kernel?

    2.Similarly in receivng path, the peripheral processor
    gives the physical address of the buffer containing
    the received packet. So, on host we need to convert
    this physical address into cached address (KSEG0ADDR).
    Before converting to cached address we need to
    invalidate the corresponding cache entries.

    Which API should be used to achieve the above
    functionlaity in Linux-2.6.18 kernel?

    currently we are using dma_cache_wback_inv() to
    achieve above two functionalities.
    Could you please suggest us the right API to be used?

    Thanks in advance.

    Regards,
    Veerasena.

    --- Ralf Baechle wrote:

    > On Mon, Oct 01, 2007 at 01:10:59PM +0100, veerasena
    > reddy wrote:
    >
    > > Is there any problem if we use the below API's in
    > > linxu-2.6.18
    > > - dma_cache_wback_inv()
    > > - dma_cache_wback()
    > > - dma_cache_inv()
    > >
    > > functionality wise, especially in r4k.c i dont see

    > any
    > > difference between the implementation of these

    > APIs.
    > >
    > > Can we apply the 2.6.24 patch to kill these APIs

    > on
    > > 2.6.18 kernel? In this case what APIs i can use

    > for
    > > writeback, invalidation or both?
    > >
    > > I couldn't find any info. related to the above API

    > in
    > > DMA-API.txt. Could you please give some pointers

    > on
    > > the usage/working of these APIs.

    >
    > dma_cache_* were never documented. They respresent
    > the earliest attempt
    > at coming up with an API that enables portable
    > drivers and it has a few
    > shortcomings and like so many early things it was
    > never formally documented,
    > so don't expect any well defined semantics. The
    > functions never got
    > formally retired probably because it somehow managed
    > to stay under the
    > radar.
    >
    > Any drivers should use the APIs documented in
    > Documentation/DMA-API.txt
    > only. The almost equivalent operation for
    > dma_cache_* would be
    > dma_sync_single and dma_sync_sg.
    >
    > Don't even dream about using dma_cache_* for
    > anything but DMA coherency.
    > They're all internal low level APIs which know
    > nothing about Linux's
    > virtual memory system.
    >
    > Anyway, it would be much easier to help you if we
    > knew what you are trying
    > to achieve with these functions.
    >
    > Ralf
    >




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