Re: [PATCH] USB: OHCI: ohci-sm501: complete URBs in BH context

[Guenter Roeck]

On 08/06/2018 01:33 AM, Christoph Hellwig wrote:
> On Sun, Aug 05, 2018 at 02:38:22PM -0700, Guenter Roeck wrote:
> > AFAICS it used to be interrupt tolerant for all but x86 up to commit
> > 6894258eda ("dma-mapping: consolidate dma_{alloc,free}_{attrs,coherent}").
> > A quick test shows that the warning is indeed not seen if I run my test
> > on v3.18.y.
> >
> > You would have to ask Christoph why it is now interrupt-intolerant for all
> > architectures.
>
> interrupt-tolerant actually is a very odd wording.  x86 itself (at least
> unless using an iommu) actually is fine with calling dma_free_* with
> interrupts disabled, but many other architectures are not.  For many
> architectures that are not cache coherent we'll have to manipulale
> the kernel page tables when freeing coherent memory, which we absolutely
> can't do from irq context.  And as usual in the kernel we try to enforce
> the same rules on everyone to allow for portable code.
>
> Also except for the odd USB use cases that abuses dma_alloc_coherent
> for the bounce buffers in device local memory you are doing something
> wrong if you call dma_alloc*/dma_free* anywhere but during device
> initialization and removal.  These are slow path interfaces for
> allocating memory for things like descriptors and similar.  Your fast
> path should be using dma_map*/dma_unmap*.

FWIW, that isn't "my" code. I just had the impunity to test it.

Cheers,
Guenter