Re: [Qemu-devel] [RFC 01/10] ide: add is_write() macro for semantic consistency

[John Snow <jsnow@redhat.com>]

On 09/13/2014 08:54 AM, Paolo Bonzini wrote:
> Il 13/09/2014 06:34, John Snow ha scritto:
>> The prepare_buf callback takes an argument named /is_write/,
>> however in core.c we are checking to see if this DMA command
>> is /is_read/. I am adding a small macro to correct this oversight.
>>
>> Impact: Nothing, yet.
>> -The prepare_buf callback is only used in ahci and pci, and both
>>   versions of this callback name the incoming argument is_write.
>> -Both functions ignore this hint currently, anyway.
>>
>> This is therefore a simple patch to avoid future mistakes.
>>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> ---
>>   hw/ide/core.c     | 2 +-
>>   hw/ide/internal.h | 2 ++
>>   2 files changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/ide/core.c b/hw/ide/core.c
>> index 191f893..3d682e2 100644
>> --- a/hw/ide/core.c
>> +++ b/hw/ide/core.c
>> @@ -718,7 +718,7 @@ void ide_dma_cb(void *opaque, int ret)
>>       n = s->nsector;
>>       s->io_buffer_index = 0;
>>       s->io_buffer_size = n * 512;
>> -    if (s->bus->dma->ops->prepare_buf(s->bus->dma, ide_cmd_is_read(s)) == 0) {
>> +    if (s->bus->dma->ops->prepare_buf(s->bus->dma, ide_cmd_is_write(s)) == 0) {
>>           /* The PRDs were too short. Reset the Active bit, but don't raise an
>>            * interrupt. */
>>           s->status = READY_STAT | SEEK_STAT;
>> diff --git a/hw/ide/internal.h b/hw/ide/internal.h
>> index 5c19f79..72d0147 100644
>> --- a/hw/ide/internal.h
>> +++ b/hw/ide/internal.h
>> @@ -338,6 +338,8 @@ enum ide_dma_cmd {
>>
>>   #define ide_cmd_is_read(s) \
>>   	((s)->dma_cmd == IDE_DMA_READ)
>> +#define ide_cmd_is_write(s) \
>> +    ((s)->dma_cmd == IDE_DMA_WRITE)
>>
>>   /* NOTE: IDEState represents in fact one drive */
>>   struct IDEState {
>>
>
> Actually the code is right (!).
>
> A read command corresponds to a DMA write.  A write or trim will read
> data from memory, so it is a DMA read.
>
> See for example how rw_buf is only used from the READ CD command, but
> passes 1 for the second argument of prepare_buf.
>
> Paolo
>

Ah, okay ... It didn't seem as if this argument was actually getting 
/used/ anywhere yet, so it wasn't really clear what it was intended to 
mean or for whom. None of the prepare_buf implementations actually seem 
to use this bit.

If you think this is correct the way it is, we should probably add some 
documentation to the prepare_buf callback to explain the nature of the 
argument to prevent someone from being helpful and trying to fix it 
again in the future.

Thanks for reviewing, this series is RFC for a reason :)