From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas Subject: Re: [RFC PATCH 2/4] pio-mapping: Add ARM support for the PIO mapping API Date: Fri, 05 Feb 2010 18:02:20 +0000 Message-ID: <1265392940.7692.123.camel@pc1117.cambridge.arm.com> References: <20100205163044.30827.10915.stgit@pc1117.cambridge.arm.com> <20100205163154.30827.6636.stgit@pc1117.cambridge.arm.com> <1265388234.14404.47.camel@mulgrave.site> <1265390403.7692.101.camel@pc1117.cambridge.arm.com> <1265391384.14404.53.camel@mulgrave.site> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:60419 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933516Ab0BESCY (ORCPT ); Fri, 5 Feb 2010 13:02:24 -0500 In-Reply-To: <1265391384.14404.53.camel@mulgrave.site> Sender: linux-arch-owner@vger.kernel.org List-ID: To: James Bottomley Cc: linux-arch@vger.kernel.org On Fri, 2010-02-05 at 17:36 +0000, James Bottomley wrote: > On Fri, 2010-02-05 at 17:20 +0000, Catalin Marinas wrote: > > On Fri, 2010-02-05 at 16:43 +0000, James Bottomley wrote: > > > On Fri, 2010-02-05 at 16:31 +0000, Catalin Marinas wrote: > > > > This patch introduces support for the PIO mapping API on the ARM > > > > architecture. It is currently only meant as an example for discussions > > > > and it can be further optimised. > > [...] > > > > diff --git a/arch/arm/include/asm/pio-mapping.h b/arch/arm/include/asm/pio-mapping.h > > > > new file mode 100644 > > > > index 0000000..d7c866a > > > > --- /dev/null > > > > +++ b/arch/arm/include/asm/pio-mapping.h > > [...] > > > > +static inline void *pio_map_single(void *addr, size_t size, > > > > + enum pio_data_direction dir) > > > > +{ > > > > + return addr; > > > > > > This API is a bit semantically nasty to use, isn't it? What we usually > > > get in the I/O path is a scatter gather list of pages and offsets (or > > > just a page in the network case). > > > > > > In a highmem kernel, we'd have to kmap the page before it actually had a > > > kernel virtual address, so now the use case of the API becomes > > > > > > vaddr = kmap_...(page) > > > pio_map_single(vaddr) > > > > > > and the reverse on unmap. > > > > > > Why not just combine the two since we always have to do them anyway and > > > do > > > > > > kmap_pio ... with the various atomic versions? > > > > For most cases, what we need looks close enough to the kmap semantics > > but this wouldn't work for the HCD case - the USB mass storage may use > > kmap() but it cannot easily be converted to the kmap_pio API since it's > > only the HCD driver that knows what kind of transfer it would do (and > > this only gets a (void *) pointer). > > OK, so I'm not very familiar with the mechanics of useb, but if it gets > a pointer to the kernel virtual address what's wrong with just doing > virt_to_page() on that? Note that virt_to_page() only really works if > the page is really a proper offset mapped kernel address, but my little > reading in drivers/usb seems to indicate it's a kmalloc buffer. According to this commit - 96983d2d8 - the USB mass storage only passes lowmem pages to the non-DMA HCD drivers, so virt_to_page() is safe. The only problem is that the transfer buffer passed to HCD may be bigger than a page (I've seen it going to 64K) and you would need to call kmap_pio() in the enqueue() function for each page, store the pointers somewhere and later (once filled) unmap them via kunmap_pio() in the dequeue() function (my temporary solution is this - http://lkml.org/lkml/2010/2/2/142). > > > > your pio_map_page is going to have to contain a kmap in the arch > > > implementation anyway ... > > > > In my approach, the only case for pio_map_page() would be for > > non-highmem pages, otherwise you would need to use pio_map_single() on > > the value returned by kmap() (as you pointed out, it doesn't look nice > > but I don't have a solution to accommodate a kmap API with the HCD > > drivers). > > So the distinction between higmemem and non-highmem is really one I'd > like to contain within the API ... otherwise we get drivers peppered > with > > if (PageHighMem(page)) { > ... > } else { > ... > } > > which leads to code proliferation and potentially more bugs. I agree. Some more thinking needed over the weekend. Thanks. -- Catalin