From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Hutchings Subject: Re: [PATCH net-next 20/22] sfc: reuse pages to avoid DMA mapping/unmapping costs Date: Wed, 22 May 2013 18:43:02 +0100 Message-ID: <1369244582.2670.32.camel@bwh-desktop.uk.level5networks.com> References: <1363030400.2608.37.camel@bwh-desktop.uk.solarflarecom.com> <1363031947.2608.57.camel@bwh-desktop.uk.solarflarecom.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: , Daniel Pieczko , linux-net-drivers To: Alex Williamson Return-path: Received: from webmail.solarflare.com ([12.187.104.25]:12844 "EHLO webmail.solarflare.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752049Ab3EVRnH (ORCPT ); Wed, 22 May 2013 13:43:07 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On Wed, 2013-05-22 at 16:29 +0000, Alex Williamson wrote: > Ben Hutchings solarflare.com> writes: > > > > > From: Daniel Pieczko solarflare.com> > > > > On POWER systems, DMA mapping/unmapping operations are very expensive. > > These changes reduce these costs by trying to reuse DMA mapped pages. [...] > > When an IOMMU is not present, the recycle ring can be small to reduce > > memory usage, since DMA mapping operations are inexpensive. > > I'm not sure I agree with the test for whether an IOMMU is present... > > > diff --git a/drivers/net/ethernet/sfc/efx.c > b/drivers/net/ethernet/sfc/efx.c > > index 1213af5..a70c458 100644 > > --- a/drivers/net/ethernet/sfc/efx.c > > +++ b/drivers/net/ethernet/sfc/efx.c > [snip] > > +void efx_init_rx_recycle_ring(struct efx_nic *efx, > > + struct efx_rx_queue *rx_queue) > > +{ > > + unsigned int bufs_in_recycle_ring, page_ring_size; > > + > > + /* Set the RX recycle ring size */ > > +#ifdef CONFIG_PPC64 > > + bufs_in_recycle_ring = EFX_RECYCLE_RING_SIZE_IOMMU; > > +#else > > + if (efx->pci_dev->dev.iommu_group) > > + bufs_in_recycle_ring = EFX_RECYCLE_RING_SIZE_IOMMU; > > + else > > + bufs_in_recycle_ring = EFX_RECYCLE_RING_SIZE_NOIOMMU; > > +#endif /* CONFIG_PPC64 */ > > Testing for an iommu_group is more of a test of (is an iommu present && does > it support the iommu api && does it support iommu groups && is the device > isolatable). That doesn't seem like what we want here (besides, it's kind > of a hacky sidestep to the API which would suggest using iommu_group_get/put > here). Since we don't try to use the iommu_group itself, those functions don't seem to be appropriate. > We could use iommu_present(&pci_bus_type), which reduces the test to (iommu > present && supports iommu api (ie. iommu_ops)). That's the test we use OOT for older kernel version. However I advised Daniel, apparently wrongly, that testing iommu_group would now be more accurate. > Better, but I think you > really care about an iommu present with dma_ops. I think we can assume that > if an iommu supports iommu_ops, it supports dma_ops, but that still leaves > out iommus that do not support iommu_ops. Do we care about those? Unfortunately the pSeries IOMMU code doesn't support iommu_ops yet, and that is precisely the case where DMA map/unmap operations are most expensive (that we've seen). > Furthermore, what about cases where an iommu is present, but unused? For > example, iommu=pt (passthrough). I'd think the driver would want to behave > as it would in the non-iommu case in that configuration. Anyway, I don't > think iommu_group is the correct test here. Thanks, Right. The real question the driver should ask is: 'will DMA-mapping/ unmapping for this device be significantly slower than DMA-syncing?' We don't yet have a way to ask that; maybe that should be added to the DMA API. Ben. -- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked.