linux-kernel - Re: [PATCH] MIPS: Flush cache after DMA_FROM_DEVICE for agressively speculative CPUs

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Date:	Thu, 14 May 2015 15:42:42 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Kevin Cernekee <cernekee@...il.com>,
	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
CC:	mina86@...a86.com,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	Zubair.Kakakhel@...tec.com, Ralf Baechle <ralf@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: Flush cache after DMA_FROM_DEVICE for agressively
 speculative CPUs

On 13/05/15 19:49, Kevin Cernekee wrote:
> On Wed, May 13, 2015 at 6:49 PM, Leonid Yegoshin
> <Leonid.Yegoshin@...tec.com> wrote:
>> Some MIPS CPUs have an aggressive speculative load and may erroneuosly load
>> some cache line in the middle of DMA transaction. CPU discards result but cache
>> doesn't. If DMA happens from device then additional cache invalidation is needed
>> on that CPU's after DMA.
>>
>> Found in test.
>>
>> Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
>> ---
>>  arch/mips/mm/dma-default.c |   10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/mips/mm/dma-default.c b/arch/mips/mm/dma-default.c
>> index 609d1241b0c4..ccf49ecfbf8c 100644
>> --- a/arch/mips/mm/dma-default.c
>> +++ b/arch/mips/mm/dma-default.c
>> @@ -67,11 +67,13 @@ static inline struct page *dma_addr_to_page(struct device *dev,
>>   * systems and only the R10000 and R12000 are used in such systems, the
>>   * SGI IP28 Indigo² rsp. SGI IP32 aka O2.
>>   */
>> -static inline int cpu_needs_post_dma_flush(struct device *dev)
>> +static inline int cpu_needs_post_dma_flush(struct device *dev,
>> +                                          enum dma_data_direction direction)
>>  {
>>         return !plat_device_is_coherent(dev) &&
>>                (boot_cpu_type() == CPU_R10000 ||
>>                 boot_cpu_type() == CPU_R12000 ||
>> +               (cpu_has_maar && (direction != DMA_TO_DEVICE)) ||
>>                 boot_cpu_type() == CPU_BMIPS5000);
> 
> Can all of these CPUs safely skip the post_dma_flush on DMA_TO_DEVICE
> (not just maar)?

Agreed that would seem like the logical thing to do. You could then just
skip the call to cpu_needs_post_dma_flush() and move the direction test
in arch/mips/mm/dma-default.c for instance?
-- 
Florian
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