Am Donnerstag, den 02.04.2015, 17:45 +0100 schrieb Russell King - ARM Linux: > On Thu, Apr 02, 2015 at 06:29:24PM +0200, Lucas Stach wrote: > > The start of the commands must always be 64bit aligned, it's the same > > for all pipes. The size can be dword aligned if the last command in the > > stream is something like SET_STATE with a length of 2. In that case one > > needs to insert another padding dword, but as it's the end of the stream > > userspace may omit that. > > > > So that more a question of policy: do we want userspace to always > > specify the size including the padding and reject streams with a size > > not double-dword aligned, or do we just fix it up in the kernel, as it's > > equally easily done there. I was a bit on the fence here and decided to > > go the "let the kernel fix things" route. > > Without really knowing the hardware, it's hard to say. However, they > seem to be designed around a 64-bit architecture, and I would not be > surprised if the front end DMA always fetches from the command buffer > in 64-bit quantities. > > You mention SET_STATE, but the same applies to NOP, WAIT and all of the > other commands for the 2D cores - the command word must always be in > the first 32-bits of each 64-bit naturally aligned word in memory. > > Given that, my feeling (and it's only a feeling) is that it would be > potentially dangerous to allow userspace to pass a buffer which isn't > aligned. As you've found, the kernel would have to align the buffer > size up to a 64-bit quantity to add the LINK command at the end anyway. > > So, I would err on the side of having userspace always do that, and > we initially enforce that on the kernel side. If we find that's too > strict, we can always relax the kernel side - and we still remain > compatible with userspace. If we do the reverse, it's harder for the > kernel to become more strict without breaking userspace. Given the > "no regressions" rule, that's something we all want to avoid. :) > Yes, seems reasonable. I'll change this to just reject non 64bit aligned sizes and change my userspace accordingly. Thanks, Lucas -- Pengutronix e.K. | Lucas Stach | Industrial Linux Solutions | http://www.pengutronix.de/ | _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
