On Thu, Jul 11, 2013 at 1:12 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wednesday 10 July 2013, Alan Stern wrote:
>> This isn't right. There are USB host controllers that use PIO, not
>> DMA. The HAS_DMA dependency should go with the controller driver, not
>> the USB core.
>>
>> On the other hand, the USB core does call various routines like
>> dma_unmap_single. It ought to be possible to compile these calls even
>> when DMA isn't enabled. That is, they should be defined as do-nothing
>> stubs.
>
> The asm-generic/dma-mapping-broken.h file intentionally causes link
> errors, but that could be changed.
>
> The better approach in my mind would be to replace code like
>
>
> if (hcd->self.uses_dma)
>
> with
>
> if (IS_ENABLED(CONFIG_HAS_DMA) && hcd->self.uses_dma) {
>
> which will reliably cause that reference to be omitted from object code,
> but not stop giving link errors for drivers that actually require
> DMA.
This can be done for drivers/usb/core/hcd.c.
But I'm a bit puzzled by drivers/usb/core/buffer.c. E.g.
void *hcd_buffer_alloc(...)
{
....
/* some USB hosts just use PIO */
if (!bus->controller->dma_mask &&
!(hcd->driver->flags & HCD_LOCAL_MEM)) {
*dma = ~(dma_addr_t) 0;
return kmalloc(size, mem_flags);
}
for (i = 0; i < HCD_BUFFER_POOLS; i++) {
if (size <= pool_max[i])
return dma_pool_alloc(hcd->pool[i], mem_flags, dma);
}
return dma_alloc_coherent(hcd->self.controller, size, dma, mem_flags);
}
which is called from usb_hcd_map_urb_for_dma():
if (hcd->self.uses_dma) {
....
} else if (hcd->driver->flags & HCD_LOCAL_MEM) {
ret = hcd_alloc_coherent(
urb->dev->bus, mem_flags,
&urb->setup_dma,
(void **)&urb->setup_packet,
sizeof(struct usb_ctrlrequest),
DMA_TO_DEVICE);
...
}
So if DMA is not used (!hcd->self.uses_dma, i.e. bus->controller->dma_mask
is zero), and HCD_LOCAL_MEM is set, we still end up calling dma_pool_alloc()?
(Naively, I'm not so familiar with the USB code) I'd expect it to use
kmalloc() instead?
So I would change it to
if (!IS_ENABLED(CONFIG_HAS_DMA) ||
(!bus->controller->dma_mask &&
!(hcd->driver->flags & HCD_LOCAL_MEM))) {
*dma = ~(dma_addr_t) 0;
return kmalloc(size, mem_flags);
}
Thanks for your clarification!
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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