On Mon, Nov 25, 2013 at 10:26:18AM -0700, Stephen Warren wrote:
> On 11/22/2013 05:34 PM, Russell King - ARM Linux wrote:
> [... various discussions re: IS_ERR, IS_ERR_OR_NULL, etc.]
>
> Russell, so if we document dma_request_slave_channel() as follows:
>
> > /*
> > * dma_request_slave_channel - try to allocate an exclusive slave channel
> > ...
> > * Returns pointer to appropriate DMA channel on success or an error pointer.
> > * In order to ease compatibility with other DMA request APIs, this function
> > * guarantees NEVER to return NULL.
> > */
>
> Are you then OK with clients doing either of e.g.:
>
> chan = dma_request_slave_channel(...);
> if (IS_ERR(chan))
> // This returns NULL or a valid handle/pointer
> chan = dma_request_channel()
> if (!chan)
> Here, chan is invalid;
> return;
> Here, chan is valid
>
> or:
>
> if (xxx) {
> chan = dma_request_slave_channel(...);
> // Convert to same error value as else{} block generates
> if (IS_ERR(chan))
> chan = NULL
> } else {
> // This returns NULL or a valid handle/pointer
> chan = dma_request_channel()
> }
> if (!chan)
> Here, chan is invalid
> return;
> Here, chan is valid
The cleaner way to write this is:
chan = dma_request_slave_channel(...);
if (IS_ERR(chan)) {
chan = dma_request_channel();
if (!chan)
return;
}
So, if you make it to this point, chan must be valid according to the
conditions on the API - you've applied the test individally to each
return function according to its documented behaviour. If it does
happen to be NULL, that's not *your* problem as a user of the API.
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