On Mon, 23 Jan 2017, Daniel Vetter <daniel@xxxxxxxx> wrote: > 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? IMO it's perfectly clear in the kernel-doc. BR, Jani. > > Thanks, Daniel -- Jani Nikula, Intel Open Source Technology Center _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
