Re: [PATCH] stmmac: explicitly zero des0 & des1 on init

[Alexey Brodkin]

Hi David,

On Mon, 2015-06-22 at 09:43 +-0300, Alexey Brodkin wrote:
+AD4- Hi David,
+AD4- 
+AD4- On Sun, 2015-06-21 at 09:29 -0700, David Miller wrote:
+AD4- +AD4- From: Alexey Brodkin +ADw-Alexey.Brodkin+AEA-synopsys.com+AD4-
+AD4- +AD4- Date: Tue, 16 Jun 2015 20:40:41 +-0300
+AD4- +AD4- 
+AD4- +AD4- +AD4- Current implementtion of descriptor init procedure only takes 
+AD4- +AD4- +AD4- care 
+AD4- +AD4- +AD4- about
+AD4- +AD4- +AD4- ownership flag. While it is perfectly possible to have underlying 
+AD4- +AD4- +AD4- 
+AD4- +AD4- +AD4- memory
+AD4- +AD4- +AD4- filled with garbage on boot or driver installation.
+AD4- +AD4- +AD4- 
+AD4- +AD4- +AD4- And randomly set flags in non-zeroed des0 and des1 fields may 
+AD4- +AD4- +AD4- lead 
+AD4- +AD4- +AD4- to
+AD4- +AD4- +AD4- unpredictable behavior of the GMAC DMA block.
+AD4- +AD4- +AD4- 
+AD4- +AD4- +AD4- Solution to this problem is as simple as explicit zeroing of both 
+AD4- +AD4- +AD4- 
+AD4- +AD4- +AD4- des0
+AD4- +AD4- +AD4- and des1 fields of all buffer descriptors.
+AD4- +AD4- +AD4- 
+AD4- +AD4- +AD4- Signed-off-by: Alexey Brodkin +ADw-abrodkin+AEA-synopsys.com+AD4-
+AD4- +AD4- 
+AD4- +AD4- If you need the memory zero initialized, use dma+AF8-zalloc+AF8-coherent().
+AD4- 
+AD4- Indeed usage of dma+AF8-zalloc+AF8-coherent() will resolve observed issue.
+AD4- But since buffer descriptors are reused extensively I would say that
+AD4- explicit zeroing of fields with flags is useful. Probably I need to 
+AD4- add
+AD4- this clarification in commit message.
+AD4- 
+AD4- And then if we do that explicit zeroing of flags and other fields 
+AD4- which
+AD4- hold data size and addresses of data buffer and the next descriptor 
+AD4- in
+AD4- chain are all get set later we may not care about allocation of 
+AD4- zeroed
+AD4- memory.

I'm wondering if my comment makes sense and should I just change commit
message or you'd prefer to still use dma+AF8-zalloc+AF8-coherent() during
driver probe?

-Alexey--
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