[PATCH v6 07/12] usb: chipidea: add a usb2 driver for ci13xxx

[Peter Chen]

> On Wednesday 01 October 2014 06:35:58 Peter Chen wrote:
> > > Subject: Re: [PATCH v6 07/12] usb: chipidea: add a usb2 driver for
> > > ci13xxx
> > >
> > > On Tuesday 30 September 2014 20:39:34 Peter Chen wrote:
> > > > Thanks, Arnd. I had not thought setting dma mask is so
> > > > complicated, yes, it should check the return value, two things to confirm:
> > > >
> > > > - dma_coerce_mask_and_coherent or dma_set_mask_and_coherent,
> the
> > > only
> > > > difference of these two API is the first one do "dev->dma_mask =
> > > >&dev- coherent_dma_mask;"
> > > > The reason you suggest choosing dma_set_mask_and_coherent is you
> > > >do  not want assign dev->dma_mask?
> > >
> > > No, that is just the current definition on ARM32 with
> > > CONFIG_ARCH_MULTIPLATFORM, and that is going to change soon to be
> DT
> > > aware.
> > > dma_set_mask_and_coherent() is supposed to check whether the
> > > platform can support the respective mask and return an error if it cannot.
> > >
> > > > - The second parameter for dma_set_mask_and_coherent is
> > > > DMA_BIT_MASK(32), is it ok?
> > > >
> > > > I just a little confused of what's the operation is "hardcoding the dma
> mask"?
> > >
> > > dma_coerce_mask_and_coherent() will hardcode the dma mask and
> > > override whatever the platform says is necessary.
> > >
> >
> > So, we should use dma_set_mask_and_coherent() in most of cases in
> > device driver, and use dma_coerce_mask_and_coherent() only  when the
> device's dma_mask is wrong?
> >
> >
> 
> dma_coerce_mask_and_coherent() should really not be used at all. Russell
> introduced it to convert drivers that were incorrectly setting the dma_mask
> pointer themselves to something slightly more palatable.
> 
> The initial dma_mask is supposed to be set by the platform for each DMA
> capable device, according to the address width of its upstream bus.
> For DT based probing, we now set it to 32-bit mask but should really set it to
> something smaller if the bus is more limited than that.
> 
> For devices created by platform code (board files), the platform should call
> platform_device_register_full() and specify the dma mask in the
> platform_device_info. Older platforms sometimes define a static platform
> device structure that has the dma mask set. This works as well but is
> discouraged for other reasons.
> 
> Drivers that require a dma mask that is either smaller than 32-bit (because of
> device specific quirks) or that know that the device is capable of larger DMA
> space and want to use that need to call dma_set_mask or
> dma_set_mask_and_coherent and check the return value.
> 
> Ideally all device drivers that want to do DMA should call set_dma_mask or
> set_dma_mask_and_coherent, even if the device has exactly a 32-bit
> connection to the upstream bus.
> 
> One more complication: if the device is not a DMA master itself but uses a dma
> engine, you should not use set_dma_mask_and_coherent() but instead use
> dma_get_mask() on the dmaengine device to query its mask, and also use that
> same pointer to pass into dma_alloc_coherent and dma_map_*.
> 
 
Thank you very much, Arnd. It makes things clear.

Peter