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SubjectRe: dma-mapping: clearing GFP_ZERO flag caused crashes of Ethernet on arc/hsdk board.
Hi Christoph, Andy

On 03/27/2018 11:11 AM, Andy Shevchenko wrote:
> On Tue, Mar 27, 2018 at 8:12 PM, Evgeniy Didin
> <> wrote:
>> Hello,
>> After commit 57bf5a8963f8 ("dma-mapping: clear harmful GFP_* flags in common code") we noticed problems with Ethernet controller on one of our platforms (namely ARC HSDK).
>> I
>> n particular we see that removal of __GFP_ZERO flag in function dma_alloc_attrs() was the culprit because in our implementation of arc_dma_alloc() we only allocate zeroed pages if
>> that flag is explicitly set by the caller. Now with unconditional removal of that flag in dma_alloc_attrs() we allocate non-zeroed pages and that seem to cause problems.
>> From
>> mentioned commit message I may conclude that architectural code is supposed to always allocate zeroed pages but I cannot find any requirement of that in kernel's documentation.
>> Coul
>> d you please point me to that requirement if that exists at all, then we'll implement a fix in our arch code like that:


> Another question why caller can't ask for zero pages explicitly?

Question to whom ? The caller can ask for it - but the problem here is generic dma
API code is clearing out GFP_ZERO and expecting arch code to memst unconditionally
- is that expected of arch code - and is documented ?

That is broken to begin with - arch dma_alloc* simply passes thru gfp flags to
page allocator and doesn't muck around with them. We could in theory but doesn't
seem like the right thing to do IMO.


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