linux-kernel - RE: [PATCH 8/9] dspbridge: add map support for big buffers

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Date:	Thu, 26 Aug 2010 08:16:24 -0500
From:	"Guzman Lugo, Fernando" <fernando.lugo@...com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ohad@...ery.com" <ohad@...ery.com>,
	"hiroshi.doyu@...ia.com" <hiroshi.doyu@...ia.com>,
	"ameya.palande@...ia.com" <ameya.palande@...ia.com>,
	"felipe.contreras@...ia.com" <felipe.contreras@...ia.com>
Subject: RE: [PATCH 8/9] dspbridge: add map support for big buffers

>________________________________________
>From: FUJITA Tomonori [fujita.tomonori@....ntt.co.jp]
>Sent: Thursday, August 26, 2010 7:56 AM
>To: Guzman Lugo, Fernando
>Cc: fujita.tomonori@....ntt.co.jp; linux-omap@...r.kernel.org; linux-kernel@...r.kernel.org; ohad@...ery.com; hiroshi.doyu@...ia.com; ameya.palande@...ia.com; >felipe.contreras@...ia.com
>Subject: RE: [PATCH 8/9] dspbridge: add map support for big buffers
>
>On Thu, 26 Aug 2010 07:38:26 -0500
>"Guzman Lugo, Fernando" <fernando.lugo@...com> wrote:
>
>> >> due to a restriction in scatter gather lists, it can
>> >> not be created a list for a buffer bigger than 1MB.
>> >> This patch is spliting big mappings into 1MB mappings.
>> >
>> >I don't read the whole thread but the above description is incorrect.
>> >
>> >Why can't you use scatter gather chaining? You can create a scatter
>> >list for a buffer larger than 1MB.
>>
>> The thing is that ARM architecture does not support SG chaining. This patch should be
>> seen like a workaround until it is fixed in iovmm module.
>
>Fix iovmm module is easier than the workaround?

I have not thought about possible fixes inside iovmm, but now I am reviewing the
patches I will figure out how to fix this inside iovmm. Since he workaround still
can fail to allocate big buffers even though there is space to allocate it.

>
>>From a quick look, arch/arm/plat-omap/iovmm.c does nothing wrong about
>scatter gather chaining. arm/arm/mm/dma-mapping.c looks fine too about
>it. Maybe just defining ARCH_HAS_SG_CHAIN works?

I can try, but it looks tricky for me. Maybe someone else can tell us why
SG chain is not supported for ARM or if there is a problem just defining 
ARCH_HAS_SG_CHAIN.

Regards,
Fernando.



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