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

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

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).

Manuel