Hi Christoph, Andy On 03/27/2018 11:11 AM, Andy Shevchenko wrote:
On Tue, Mar 27, 2018 at 8:12 PM, Evgeniy Didin <Evgeniy.Didin@xxxxxxxxxxxx> 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:
[snip]
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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