Re: [PATCH 2/2] usb: cdns3: Optimize DMA request buffer allocation

[Christoph Hellwig]

On Tue, Mar 09, 2021 at 10:18:43AM +0000, Sanket Parmar wrote:
> > On Tue, Mar 09, 2021 at 06:19:40AM +0100, Sanket Parmar wrote:
> > > dma_alloc_coherent() might fail on the platform with a small DMA region.
> > >
> > > To avoid such failure in cdns3_prepare_aligned_request_buf(),
> > > dma_alloc_coherent() is replaced with kmalloc and dma_map API to
> > > allocate aligned request buffer of dynamic length.
> > 
> > dma_alloc_noncoherent is the proper API instead of using kmalloc, which
> > can lead to unaddressable memory that might require bounce buffering.
> 
> cdns3 device required DMA coherent buffer to perform operations. So 
> dma_alloc_noncoherent will not help here.
> 
> Also all gadget classes(except g_ether) use kmalloc to allocated request buffer,
> and device driver uses usb_gadget_map_request_by_dev to map the request
> buffer. Similar approach is used to allocate aligned buffer. 

If you can use kmalloc and dma_map_single you can use
dma_alloc_noncoherent per definition.