From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757712AbdJMKsz (ORCPT ); Fri, 13 Oct 2017 06:48:55 -0400 Received: from foss.arm.com ([217.140.101.70]:59160 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751122AbdJMKsx (ORCPT ); Fri, 13 Oct 2017 06:48:53 -0400 Subject: Re: [PATCH] xhci: Set DMA parameters appropriately To: Marek Szyprowski , mathias.nyman@intel.com, gregkh@linuxfoundation.org Cc: linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, hch@lst.de, iommu@lists.linux-foundation.org References: <347776be09eb0ca947c1c8ced46ee3ca92d92e28.1507730115.git.robin.murphy@arm.com> <1cde45f2-cdd5-6210-dc00-49637132b897@samsung.com> From: Robin Murphy Message-ID: <6b85dd6d-9728-e4f7-30f2-55d9f6f10372@arm.com> Date: Fri, 13 Oct 2017 11:48:50 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 MIME-Version: 1.0 In-Reply-To: <1cde45f2-cdd5-6210-dc00-49637132b897@samsung.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Marek, On 13/10/17 09:15, Marek Szyprowski wrote: > Hi Robin, > > 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 >> --- >>   drivers/usb/host/xhci.c | 4 ++++ >>   drivers/usb/host/xhci.h | 3 +++ >>   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) >>           dma_set_coherent_mask(dev, DMA_BIT_MASK(32)); >>       } >>   +    dev->dma_parms = &xhci->dma_parms; >> +    dma_set_max_seg_size(dev, SZ_64K); >> +    dma_set_seg_boundary(dev, SZ_64K - 1); >> + >>       xhci_dbg(xhci, "Calling HCD init\n"); >>       /* Initialize HCD and host controller data structures. */ >>       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 { >>       struct dma_pool    *small_streams_pool; >>       struct dma_pool    *medium_streams_pool; >>   +    /* DMA alignment restrictions */ >> +    struct device_dma_parameters dma_parms; >> + >>       /* Host controller watchdog timer structures */ >>       unsigned int        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. > 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. Robin.