Re: [PATCH] MIPS: Alchemy: fix dbdma ring destruction memory debugcheck.

[Manuel Lauss <manuel.lauss@googlemail.com>]

David,

On Tue, Jan 26, 2010 at 6:58 PM, Manuel Lauss
<manuel.lauss@googlemail.com> wrote:
> On Tue, Jan 26, 2010 at 6:44 PM, David Daney <ddaney@caviumnetworks.com> 
> wrote:
>> Manuel Lauss wrote:
>>>
>>> DBDMA descriptors need to be located at 32-byte aligned addresses;
>>> however kmalloc rarely delivers such addresses.  The dbdma code
>>> works around that by allocating 63 bytes and re-aligning the
>>> descriptor base afterwards.  Hoewever when freeing memory it does
>>> not account for this adjustment and trips the kfree debugcheck:
>>>
>>
>> Correct me if I am wrong, but don't kmalloc et al. return blocks aligned
>>  boundaries of the size rounded up the the next power of two?  So if you
>> need 32-byte aligned addresses, just use a size value of 32 or greater.  You
>> wouldn't have to add 63 and do masking and remember the membase value as you
>> do in the patch.
>
> The description is not completely correct (I suck a writing those):
> It allocates a number
> of descriptor entries (64 bytes each) specified by the driver in a single 
> block:
>
>        desc_base = (u32)kmalloc(entries * sizeof(au1x_ddma_desc_t),
>                                 GFP_KERNEL|GFP_DMA);
>        if (desc_base == 0)
>                return 0;
>
>        if (desc_base & 0x1f) {
>
> So far the 3 users I have (mmc, spi, audio) always return true on the above
> check (2 descriptors for audio for instance).

... but only if CONFIG_DEBUG_SLAB is enabled.  You are correct in that
kmalloc() returns aligned blocks with a non-debug slab allocator and this
fix becomes superfluous.   I'll try to confirm this with the other
allocators and
resend with a fixed description.

Thank you for the pointer!
     Manuel Lauss