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    SubjectRe: Linux 3.4-rc4
    (re-adding Ben to the Cc because he was apparently dropped somewhere in
    this thread)

    On 2012-05-01 09:23 -0400, Nick Bowler wrote:
    > On 2012-04-30 11:07 +0200, Maarten Maathuis wrote:
    > > On Mon, Apr 30, 2012 at 12:37 AM, Dmitry Torokhov
    > > <dmitry.torokhov@gmail.com> wrote:
    > > > On Sat, Apr 28, 2012 at 11:33:50AM -0400, Nick Bowler wrote:
    > > >> On 2012-04-28 02:19 -0400, Alex Deucher wrote:
    > > >> > On Fri, Apr 27, 2012 at 8:39 PM, Nick Bowler <nbowler@elliptictech.com> wrote:
    > > >> > > While tracking down the black screen issue, I've been having the monitor
    > > >> > > directly connected to the video card the whole time, but now when I'm
    > > >> > > connected through my KVM switch (an IOGear GCS1804), it appears that
    > > >> > > something's going wrong with reading the EDID, because the available
    > > >> > > modes are all screwed up (both console and X decide they want to drive
    > > >> > > the display at 1024x768).
    > [...]
    > > >> > > Also, looking at /sys/class/drm/card0-VGA-1/edid I see that it
    > > >> > > is empty on 3.4-rc4+ and it is correct on 3.2.15.  Things seem
    > > >> > > to work OK when the KVM is not involved.
    > > >> >
    > > >> > Were you ever able to fetch a EDID with the KVM involved?  KVMs are
    > > >> > notorious for not connecting the ddc pins.
    > > >>
    > > >> Yes, it works on 3.2.15 as described above.
    > > >
    > > > I have the same (or similar) KVM (not in the office at the moment) and I
    > > > can confirm that with newer kernels EDID fecthing in flaky. It's 50/50
    > > > if EDED retrieval succeeds or if it fails with:
    > > >
    > > > Apr 26 13:06:57 dtor-d630 kernel: [13464.936336] [drm:drm_edid_block_valid] *ERROR* EDID checksum is invalid, remainder is 208
    > [...]
    > > > Earlier kernels were able to retrieve EDEDs reliably.
    >
    > FWIW, for me EDID failure on new kernels is 100% reproducible, and there
    > are no such checksum errors in the log. It's just missing.
    >
    > > Just a crazy thought, but didn't we change some timings related to
    > > EDID retrieval? To make it faster.
    >
    > OK, this time bisecting started off relatively smoothly (doing the same
    > "backwards" bisect on the branch-o-reverts as last time), but then my
    > disk died halfway through...
    [...]

    OK, system is back online and I finished the bisection. The commit that
    broke it for me is the following, and reverting it on top of 3.3.4 + the
    "make VGA work at all" patch fixes this particular issue for me.

    commit f553b79c03f0dbd52f6f03abe8233a2bef8cbd0d
    Author: Ben Skeggs <bskeggs@redhat.com>
    Date: Wed Dec 21 18:09:12 2011 +1000

    drm/nouveau/i2c: handle bit-banging ourselves

    i2c-algo-bit doesn't actually work very well on one card I have access to
    (NVS 300), random single-bit errors occur most of the time - what we're
    doing now is closer to what xf86i2c.c does.

    The original plan was to figure out why i2c-algo-bit fails on the NVS 300,
    and fix it. However, while investigating I discovered i2c-algo-bit calls
    cond_resched(), which makes it a bad idea for us to be using as we execute
    VBIOS scripts from a tasklet, and there may very well be i2c transfers as
    a result.

    So, since I already wrote this code in userspace to track down the NVS 300
    bug, and it's not really much code - lets use it.

    Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

    Cheers,
    --
    Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)

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