Message-ID: <20220926163408.110bbc97@computer>
Date:   Mon, 26 Sep 2022 16:34:08 +0200
From:   Simon Rettberg <simon.rettberg@...uni-freiburg.de>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, Lyude Paul <lyude@...hat.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Alex Deucher <alexander.deucher@....com>
Subject: Re: [PATCH RESEND] drm/display: Don't assume dual mode adaptors
 support i2c sub-addressing

On Mon, 26 Sep 2022 15:38:43 +0300
 Ville Syrjälä <ville.syrjala@...ux.intel.com> wrote:

> On Mon, Sep 26, 2022 at 12:40:17PM +0200, Simon Rettberg wrote:
> > Current dual mode adaptor ("DP++") detection code assumes that all
> > adaptors support i2c sub-addressing for read operations from the
> > DP-HDMI adaptor ID buffer.  It has been observed that multiple
> > adaptors do not in fact support this, and always return data
> > starting at register 0.  On affected adaptors, the code failed to
> > read the proper registers that would identify the device as a type
> > 2 adaptor, and handled those as type 1, limiting the TMDS clock to
> > 165MHz. Fix this by always reading the ID buffer starting from
> > offset 0, and discarding any bytes before the actual offset of
> > interest.
> > 
> > Signed-off-by: Simon Rettberg <simon.rettberg@...uni-freiburg.de>
> > Reviewed-by: Rafael Gieschke <rafael.gieschke@...uni-freiburg.de>
> > ---
> > (Resend because of no response, probably my fault since I ran
> > get_maintainers on a shallow clone and missed a bunch of people)
> > 
> > We had problems with multiple different "4k ready" DP++ adaptors
> > only resulting in 1080p resolution on Linux. While one of them
> > turned out to actually just be a type1 adaptor, the others,
> > according to the data retreived via i2cdump, were in fact proper
> > type2 adaptors, advertising a TMDS clock of 300MHz. As it turned
> > out, none of them supported sub-addressing when reading from the
> > DP-HDMI adaptor ID buffer via i2c. The existing code suggested that
> > this is known to happen with "broken" type1 adaptors, but
> > evidently, type2 adaptors are also affected. We tried finding
> > authoritative documentation on whether or not this is allowed
> > behavior, but since all the official VESA docs are paywalled, the
> > best we could come up with was the spec sheet for Texas
> > Instruments' SNx5DP149 chip family.[1] It explicitly mentions that
> > sub-adressing is supported for register writes, but *not* for reads
> > (See NOTE in section 8.5.3). Unless TI blatantly and openly decided
> > to violate the VESA spec, one could take that as a strong hint that
> > sub-addressing is in fact not mandated by VESA.  
> 
> I don't think that would pass the dual mode CTS for type2 adaptors
> since it explicitly calls for reading individual bytes from various
> offsets.
> 
> The actual dual mode spec specifies things rather poorly. Technically
> it doesn't even specify the write protocol, and the read protocol is
> only specified in the form of an example read of the HDMI ID buffer.
> There it says the offset write is optional for the master, but
> mandatory for the slave to ack. It neither explicitly allows nor
> disallows the ack+ignore behaviour, but IIRC there is some
> text in there that suggests that type1 adaptors might ignore it.

Interesting, but poor spec would explain why it's not implemented by
at least three such chips. That's the TI one (we don't actually have
it, but the data sheet above seems quite clear), and the two we
confirmed it with: the PS8409(A), and the LT8611.
So either way it might make sense to handle this. Since the first
submission of this patch I also took the time to check it on
Windows 10, and both adaptors make Windows list 4k resolutions with
both the intel iGPU and an nvidia card.

Here are the two dumps for completeness:

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: 44 50 2d 48 44 4d 49 20 41 44 41 50 54 4f 52 04    DP-HDMI ADAPTOR?
10: a0 00 1c f8 50 53 38 34 30 39 a2 00 00 78 08 ff    ?.??PS8409?..x?.

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: 44 50 2d 48 44 4d 49 20 41 44 41 50 54 4f 52 04    DP-HDMI ADAPTOR?
10: a0 ff ff ff 4c 54 38 36 31 31 a2 00 00 78 0f 00    ?...LT8611?..x?.

> 
> > 
> > [1] https://www.ti.com/lit/ds/symlink/sn75dp149.pdf
> > 
> >  .../gpu/drm/display/drm_dp_dual_mode_helper.c | 52
> > ++++++++++--------- 1 file changed, 28 insertions(+), 24
> > deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c
> > b/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c index
> > 3ea53bb67..6147da983 100644 ---
> > a/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c +++
> > b/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c @@ -63,23
> > +63,42 @@ ssize_t drm_dp_dual_mode_read(struct i2c_adapter *adapter,
> >  			      u8 offset, void *buffer, size_t size)
> >  {
> > +	int ret;
> > +	u8 zero = 0;
> > +	char *tmpbuf;
> > +	/*
> > +	 * As sub-addressing is not supported by all adaptors,
> > +	 * always explicitly read from the start and discard
> > +	 * any bytes that come before the requested offset.
> > +	 * This way, no matter whether the adaptor supports it
> > +	 * or not, we'll end up reading the proper data.
> > +	 */
> >  	struct i2c_msg msgs[] = {
> >  		{
> >  			.addr = DP_DUAL_MODE_SLAVE_ADDRESS,
> >  			.flags = 0,
> >  			.len = 1,
> > -			.buf = &offset,
> > +			.buf = &zero,
> >  		},
> >  		{
> >  			.addr = DP_DUAL_MODE_SLAVE_ADDRESS,
> >  			.flags = I2C_M_RD,
> > -			.len = size,
> > -			.buf = buffer,
> > +			.len = size + offset,
> > +			.buf = NULL,
> >  		},
> >  	};
> > -	int ret;
> >  
> > +	tmpbuf = kmalloc(size + offset, GFP_KERNEL);
> > +	if (!tmpbuf)
> > +		return -ENOMEM;
> > +
> > +	msgs[1].buf = tmpbuf;
> >  	ret = i2c_transfer(adapter, msgs, ARRAY_SIZE(msgs));
> > +	if (ret == ARRAY_SIZE(msgs))
> > +		memcpy(buffer, tmpbuf + offset, size);
> > +
> > +	kfree(tmpbuf);  
> 
> Could optimize a bit here and avoid the temp buffer when
> the original offset is 0.

Was thinking about that too while writing the patch, but decided
to keep it as straight forward as possible for the initial submission;
it's also not really performance critical, should be called a few
times when the adaptor is plugged in, and probably just once with
offset 0.
It also didn't feel nice to have the "if (ret == ARRAY_SIZE(msgs))"
check duplicated for the memcpy, to avoid copying potentially
uninitialised memory into the output buffer. I didn't see how this
would lead to an information leak to user space with the current
code base, but better safe than sorry? :)
The alternative is to move the memcpy down and merge it with the
other if-block, but then we'd need a cleanup label at the bottom
to do the kfree in the error case that comes before that...
But I'll happily refine that further and submit a v2 if desired.

> 
> > +
> >  	if (ret < 0)
> >  		return ret;
> >  	if (ret != ARRAY_SIZE(msgs))
> > @@ -208,18 +227,6 @@ enum drm_dp_dual_mode_type
> > drm_dp_dual_mode_detect(const struct drm_device *dev, if (ret)
> >  		return DRM_DP_DUAL_MODE_UNKNOWN;
> >  
> > -	/*
> > -	 * Sigh. Some (maybe all?) type 1 adaptors are broken and
> > ack
> > -	 * the offset but ignore it, and instead they just always
> > return
> > -	 * data from the start of the HDMI ID buffer. So for a
> > broken
> > -	 * type 1 HDMI adaptor a single byte read will always give
> > us
> > -	 * 0x44, and for a type 1 DVI adaptor it should give 0x00
> > -	 * (assuming it implements any registers). Fortunately
> > neither
> > -	 * of those values will match the type 2 signature of the
> > -	 * DP_DUAL_MODE_ADAPTOR_ID register so we can proceed with
> > -	 * the type 2 adaptor detection safely even in the presence
> > -	 * of broken type 1 adaptors.
> > -	 */
> >  	ret = drm_dp_dual_mode_read(adapter,
> > DP_DUAL_MODE_ADAPTOR_ID, &adaptor_id, sizeof(adaptor_id));  
> 
> Another optimization opportunity here to maybe combine the HDMI ID
> buffer read with this one. Could perhaps just read the full 32 bytes
> static capabilities section. But this one should probably be left for
> a separate patch. Ideally I guess we'd also combine the max TMDS clock
> read with this one. But for that we'd need to return more than the
> single enum drm_dp_dual_mode_type from this function.

Pretty much same as above, keep v1 simple, but I noticed that too.
If that's going to be another patch anyways, it might make sense
if that's done by someone more familiar with that code in general
(basically had to research all this DP++/i2c stuff from scratch).
But I could give it a spin.

> 
> >  	drm_dbg_kms(dev, "DP dual mode adaptor ID: %02x (err
> > %zd)\n", adaptor_id, ret); @@ -233,11 +240,10 @@ enum
> > drm_dp_dual_mode_type drm_dp_dual_mode_detect(const struct
> > drm_device *dev, return DRM_DP_DUAL_MODE_TYPE2_DVI; }
> >  		/*
> > -		 * If neither a proper type 1 ID nor a broken type
> > 1 adaptor
> > -		 * as described above, assume type 1, but let the
> > user know
> > -		 * that we may have misdetected the type.
> > +		 * If not a proper type 1 ID, still assume type 1,
> > but let
> > +		 * the user know that we may have misdetected the
> > type. */
> > -		if (!is_type1_adaptor(adaptor_id) && adaptor_id !=
> > hdmi_id[0])
> > +		if (!is_type1_adaptor(adaptor_id))
> >  			drm_err(dev, "Unexpected DP dual mode
> > adaptor ID %02x\n", adaptor_id); 
> >  	}
> > @@ -343,10 +349,8 @@
> > EXPORT_SYMBOL(drm_dp_dual_mode_get_tmds_output);
> >   * @enable: enable (as opposed to disable) the TMDS output buffers
> >   *
> >   * Set the state of the TMDS output buffers in the adaptor. For
> > - * type2 this is set via the DP_DUAL_MODE_TMDS_OEN register. As
> > - * some type 1 adaptors have problems with registers (see comments
> > - * in drm_dp_dual_mode_detect()) we avoid touching the register,
> > - * making this function a no-op on type 1 adaptors.
> > + * type2 this is set via the DP_DUAL_MODE_TMDS_OEN register.
> > + * Type1 adaptors do not support any register writes.
> >   *
> >   * Returns:
> >   * 0 on success, negative error code on failure
> > -- 
> > 2.35.1  
> 

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