Re: [Qemu-devel] [PATCH] block: mask NOR flash buffered write length

[Peter Maydell <peter.maydell@linaro.org>]

On 18 October 2013 12:38, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Thu, Oct 17, 2013 at 07:30:02PM -0700, Roy Franz wrote:
>> For buffered writes, mask the length with the maximum supported
>> length.  This is required for block writes to work on the ARM vexpress
>> platform, where the flash interface is 32 bits wide.  For buffered writes
>> to the 2 16 bit flashes on the interface, the length is repeated in each
>> 16 bit word, and without this mask the two lengths are interpreted
>> as a single 32 bit value that is very large.

>> @@ -378,6 +378,7 @@ static void pflash_write(pflash_t *pfl, hwaddr offset,
>>
>>              break;
>>          case 0xe8:
>> +            value &= pfl->writeblock_size - 1;
>
> This patch feels weird.   Should the 32-bit interface width be truncated
> down to 16 bits before dispatching pflash_write()?
>
> It's not clear to me that truncating just in this specific case is
> correct.  But then I don't know the hardware :).

I think the problem may be that we're incorrectly modelling the
hardware's "2 side-by-side 16 bit wide flash chips" as "one 32 bit
wide flash chip", because our flash device code doesn't support
doing the former.

Probably instead of a single "width" property we should have two,
similar to the device tree binding's pair:
 - bank-width : Width (in bytes) of the bank.  Equal to the
   device width times the number of interleaved chips.
 - device-width : (optional) Width of a single mtd chip.  If
   omitted, assumed to be equal to 'bank-width'.

However I'm not very familiar with how flash hardware works...

-- PMM