[PATCH] scatterlist: define SG chain for arm architecture

[Guzman Lugo, Fernando]

On Thu, Dec 2, 2010 at 11:41 AM, Guzman Lugo, Fernando
<fernando.lugo at ti.com> wrote:
> On Sun, Oct 3, 2010 at 6:34 AM, FUJITA Tomonori
> <fujita.tomonori at lab.ntt.co.jp> wrote:
>> On Sun, 3 Oct 2010 12:28:39 +0100
>> Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
>>
>>> On Sun, Oct 03, 2010 at 08:02:52PM +0900, FUJITA Tomonori wrote:
>>> > On Fri, 1 Oct 2010 17:39:58 +0100
>>> > Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
>>> >
>>> > > On Fri, Oct 01, 2010 at 11:28:41AM -0500, Fernando Guzman Lugo
>> wrote:
>>> > > > Define SG chain for ARM architecture in order to work
>>> > > > with big buffers. This patch is needed in order to
>>> > > > iovmm users can pass a big buffer to map it and ARM
>>> > > > architecture has not issues using SG chain.
>>> > > >
>>> > > > This change was suggested by FUJITA Tomonori.
>>> > >
>>> > > Have you audited all places where SG lists are used in the ARM
>>> > > architecture to ensure that SG chain support is safe?  (That
>>> > > goes for ARM generic code and ARM specific drivers.)
>>> >
>>> > As far as I know, only SCSI actually uses SG chain feature (that is,
>>> > sending chained scatter lists). All SCSI drivers were converted when
>>> > we added SG chain feature. So what you need to care about is the dma
>>> > code.
>>> >
>>> > There are only several files that actually use scatter list under
>>> > arch/arm.
>>> >
>>> > >From a quick look, arch/arm/mm/dma-mapping.c,
>>> > arch/arm/common/dmabounce.c, arch/arm/kernel/dma.c, and
>>> > arch/arm/include/asm/dma-mapping.h looks fine.
>>> >
>>> > arm/mach-imx/dma-v1.c also looks like to use chained sg API properly
>>> > (but this isn't used with SCSI, right?).
>>> >
>>> > Seems that arch/arm/plat-omap/iovmm.c can handle the chained SG
>>> > too. It doesn't allocate the chained SG now though.
>>>
>>> There's also RiscPC and its SCSI drivers could well be an issue for SG
>>> chaining.  They have to manually save the SCSI 'data pointer' (down to
>>> the byte position in the SG chain and be able to restore a previously
>>> saved point.)  This code was written before SG chaining ever appeared,
>>> and although it has been partly fixed up, it's probably buggy if SG
>>> chaining were to be enabled.
>>
>> What SCSI drviers we need to fix?
>
> Russell,
>
> Any update with this? I hope it can be merged for 2.6.38.


Russell,

The motivation for this patch is:

The tidspbridge driver running in omap platform (omap3) is migrating
to use the iommu and iovmm modules form DMM propose. In order to map
user-side buffers into DSP virtual address we use the function
iommu_vmap() in iovmm.c file, the third parameter of that funcion is
of struct sg_table type, which contains all the pages that will be
mapped.

There is a restriction about the size of the sg list, that cannot be
bigger than PAGE_SIZE, so that the number of pages cannot be bigger
than SG_MAX_SINGLE_ALLOC. Therefore there is a restriction in the size
of the buffer which iommu_vmap() can map. However MM apps using
tidspbridge maps big buffers most of the time, so it is not possible
to use  iommu_vmap() if it cannot map big buffer. Enabling SG chain
for ARM resolves that problem.

If you have a better solution please let me know. Otherwise I will
follow your comment about make it a Kconfig entry to enable SG chain
for ARM.


Thanks a lot,
Fernando.

>
>>
>>
>>> If it's just OMAP users who wish to enable SG chaining, then the sane
>>> thing to do would be to make the definition of the macro conditional
>>> on OMAP.
>>
>> Yeah, that should work.
>
> some like this in scatterlist.h is ok?
>
> #ifdef CONFIG_ARCH_OMAP
> #define ARCH_HAS_SG_CHAIN
> #endif
>
>
> Regards,
> Fernando.
>>
>