> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
