On Mon, Jan 23, 2017 at 03:06:36PM +0100, Andrea Merello wrote: > On Mon, Jan 23, 2017 at 1:36 PM, Boris Brezillon > <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote: > > On Mon, 23 Jan 2017 13:12:12 +0100 > > Andrea Merello <andrea.merello@xxxxxxxxx> wrote: > > > >> On Mon, Jan 23, 2017 at 12:32 PM, Jani Nikula > >> <jani.nikula@xxxxxxxxxxxxxxx> wrote: > >> > On Mon, 23 Jan 2017, Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote: > >> >> Hi Andrea, > >> >> > >> >> On Mon, 23 Jan 2017 11:00:02 +0100 > >> >> Andrea Merello <andrea.merello@xxxxxxxxx> wrote: > >> >> > >> >>> From: Andrea Merello <andrea.merello@xxxxxxxxx> > >> >>> > >> >>> The standard DRM function to get the edid from the i2c bus performs > >> >>> (at least) two transfers. > >> >>> > >> >>> By experiments it seems that the sii9022a have problems with the > >> >>> 2nd I2C start, at least unless a wait is introduced detween the > >> >> > >> >> ^ between > >> >> > >> >>> two transfers. > >> >>> > >> >>> So we perform one single I2C transfer, and if the transfer must be > >> >>> split, then we introduce a delay. > >> >> > >> >> That's not exactly what this patch does: you're introducing a delay > >> >> between each retry. So, if the transceiver really requires a delay > >> >> between each transfer, you'll have to retry at least once on the 2nd > >> >> transfer. > >> >> > >> >> I guess a better solution would be to add a delay even in case of > >> >> success, or maybe modify drm_do_get_edid() to optionally wait for a > >> >> specified time between each transfer. > >> > > >> > Is the problem related specifically to EDID reads, or generally to I2C > >> > transfers? Perhaps this should be fixed at the adapter master_xfer layer > >> > instead? Does the master_xfer perhaps return -EAGAIN, have you looked > >> > into i2c_adapter timeout member? (See __i2c_transfer in i2c-core.c.) > >> > > >> > The intention of drm_do_get_edid() is to facilitate *alternative* > >> > methods to doing I2C, not to workaround quirks like this. > > > > Yes, I tend to agree with that. > > > >> > >> This problem seems not related to the I2C master itself. I saw the I2C > >> master to TX correctly by looking at the I2C bus before the sii9022a > >> with the scope, and I saw the same I2C transfer to be corrupted after > >> the sii9022a; actually the sii9022a seems to gate the bus damaging the > >> I2C start of the 2nd transfer, unless we place this delay. > >> > >> This happened with two different I2C master (the one on the SoC as > >> well as another different one implemented on FPGA). > > > > Actually, I faced the same problem on an at91-based platform, and came > > up with pretty much the same fix (add a delay after each transfer), > > except mine was uglier and I never took the time to clean it up and > > submit it. > > > >> > >> So IMHO this quirk should be done in the sii902x dirver. Said that, I > >> admit that I've not looked at i2c_adapter timeout member or other > >> details in upper layers. > > > > Well, let's sum up the situation: the sii902x device is acting both as > > a regular i2c device and as an i2c 'pass-through' device. In the > > pass-through mode, the si902x has a new constraint: the i2c transfers > > have to be separated by an idle period. > > > > Maybe we should expose the pass-through mode as separate i2c adapter, > > that would be registered when entering pass-through mode and > > de-registered when leaving this mode. > > This way, we can implement the ->master_xfer() as we need (add a delay > > after each xfer), and still use the generic drm_get_edid(). > > > > What do you think? > > IMHO this could be a good way to go.. Although honestly the current > quirk doesn't seem so much bad to me :) I agree with Jani that using drm_do_get_edid is the wrong thing to do here, it's really for cases where you have a hw edid-fetch engine that entirely hides the underlying i2c bus. If you do have the i2c bus exposed, then the right place to fix it is at the i2c_adapter level. There's a pile of userspace tools that also probe the ddc i2c bus (there's not just an edid there), and if you don't fix this at the i2c_adapter level those would still be broken. Since this seems unclear, can you pls also do a patch for the kerneldoc for the drm_do_get_edid function, so it's clear when and when not this should be used? Thanks, Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
