On Sun, Jan 18, 2015 at 01:18:51PM +0200, Laurent Pinchart wrote: > Hi Alexandre, > > On Sunday 18 January 2015 15:54:34 Alexandre Courbot wrote: > > On 01/16/2015 08:18 AM, Laurent Pinchart wrote: [...] > > > The second way is to implement a mechanism to let drivers signal that they > > > want to handle DMA mappings themselves. As the mappings need in the > > > general case to be created before the probe function is called > > > > Sorry for being ignorant here, but why is that? > > Because a driver can call the DMA mapping API in its probe function, to > allocate DMA coherent memory for instance. We need to ensure that the DMA > mapping IOMMU has set up the required IOMMU ops by that time. As explained > above I don't like the idea of sprinkling explicit calls to initialize IOMMU > support in the vast majority of drivers, especially when they shouldn't be > IOMMU-aware, so we then need to initialize everything that is needed before > the first call to the DMA mapping API.
The original patch that Hiroshi posted based on my comments was to have the driver core call iommu_attach(), which would then set up everything needed right before calling into the driver's ->probe(). This works quite nicely because by definition the driver can't allocate any DMA before ->probe(). And, like you said, this allows deferred probe to be used. To me it's so obviously the right solution that I remain flabbergasted with how much resistance it's received (or how much it's being ignored). Thierry
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