Re: [PATCH] xhci: Set DMA parameters appropriately

[Marek Szyprowski]

Hi Robin,

On 2017-10-13 12:48, Robin Murphy wrote:
> On 13/10/17 09:15, Marek Szyprowski wrote:
> > On 2017-10-11 15:56, Robin Murphy wrote:
> > > xHCI requires that data buffers do not cross 64KB boundaries (and are
> > > thus at most 64KB long as well) - whilst xhci_queue_{bulk,isoc}_tx()
> > > already split their input buffers into individual TRBs as necessary,
> > > it's still a good idea to advertise the limitations via the standard DMA
> > > API mechanism, so that most producers like the block layer and the DMA
> > > mapping implementations can lay things out correctly to begin with.
> > >
> > > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> > > ---
> > >  Â drivers/usb/host/xhci.c | 4 ++++
> > >  Â drivers/usb/host/xhci.h | 3 +++
> > >  Â 2 files changed, 7 insertions(+)
> > >
> > > diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
> > > index 74b4500641c2..1e7e1e3d8c48 100644
> > > --- a/drivers/usb/host/xhci.c
> > > +++ b/drivers/usb/host/xhci.c
> > > @@ -4883,6 +4883,10 @@ int xhci_gen_setup(struct usb_hcd *hcd,
> > > xhci_get_quirks_t get_quirks)
> > >  ÂÂÂÂÂÂÂÂÂ dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
> > >  ÂÂÂÂÂ }
> > >  Â +ÂÂÂ dev->dma_parms = &xhci->dma_parms;
> > > +ÂÂÂ dma_set_max_seg_size(dev, SZ_64K);
> > > +ÂÂÂ dma_set_seg_boundary(dev, SZ_64K - 1);
> > > +
> > >  ÂÂÂÂÂ xhci_dbg(xhci, "Calling HCD init\n");
> > >  ÂÂÂÂÂ /* Initialize HCD and host controller data structures. */
> > >  ÂÂÂÂÂ retval = xhci_init(hcd);
> > > diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
> > > index 7ef69ea0b480..afcae4cc908d 100644
> > > --- a/drivers/usb/host/xhci.h
> > > +++ b/drivers/usb/host/xhci.h
> > > @@ -1767,6 +1767,9 @@ struct xhci_hcd {
> > >  ÂÂÂÂÂ struct dma_poolÂÂÂ *small_streams_pool;
> > >  ÂÂÂÂÂ struct dma_poolÂÂÂ *medium_streams_pool;
> > >  Â +ÂÂÂ /* DMA alignment restrictions */
> > > +ÂÂÂ struct device_dma_parameters dma_parms;
> > > +
> > >  ÂÂÂÂÂ /* Host controller watchdog timer structures */
> > >  ÂÂÂÂÂ unsigned intÂÂÂÂÂÂÂ xhc_state;
> > >    
> > Are you sure that xhci_hcd life time is proper to keep dma_parms? It looks
> > that when driver gets removed and xhci_hcd is freed, the dma_parms will
> > point to freed memory. Maybe it would make sense to clear dev->dma_parms
> > somewhere or definitely change the way dma_parms are allocated?
> AFAICS it lives until the last usb_put_hcd() call, which is pretty much
> the last thing in the drivers' .remove paths, so it looks to follow the
> standard paradigm evidenced by other dma_parms users. Any dangling
> pointer after the driver has been unbound will be reinitialised by a
> subsequent probe, and anyone using an unbound device for DMA API calls
> is very wrong anyway.

Okay.

> > On the other hand 64K is the default segment size if no dma_parms are
> > provided, so there is very little value added by this patch.
> I prefer to explicitly set the segment size for cleanliness and to
> emphasize the difference between "whatever the default value is is fine"
> and "we have a real hardware limit of 64K". What really matters here
> though is the non-default segment boundary mask - that's the motiviation
> for the patch.

Okay. What about other type of USB HCD (EHCI, OHCI, UHCI)?

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland