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- <title>i945 backlight in libreboot</title>
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- <header>
- <h1 id="pagetop">i945 backlight controls on X60/T60</h1>
- </header>
-
- <p>
- <a href="index.html">back to index</a>
- </p>
-
-<hr/>
-
- <h2>Contents</h2>
- <ul>
- <li><a href="#x60_native_notes">X60 native graphics initialization (backlight controls)</a></li>
- <li><a href="#t60_native_notes">T60 native graphics initialization (backlight controls)</a></li>
- </ul>
-
-<hr/>
-
- <h1 id="x60_native_notes">X60 native graphics initialization (with backlight controls)</h1>
- <p>
- <b>The fix below was done on 5320/6 (review.coreboot.org) under src/mainboard/lenovo/x60/devicetree.cb</b>
- </p>
- <p>
- Works on the ThinkPad X60, X60s and X60 Tablet. This change is also included by default in libreboot 6th release.
- </p>
- <p>
- <b>Add backlight controls:</b> in <i>src/mainboard/lenovo/x60/devicetree.cb</i>, change <b>gpu_backlight</b> to <b>0x879F879E</b>
- </p>
- <p>
- That's all! <b>This was also backported into libreboot 5th release (line 1233 in src/mainboard/lenovo/x60/i915io.c)</b>. GNUtoo (Denis Carikli)
- told me about the register <b>BLC_PWM_CTL</b> and that you could set it to control backlight. I read that address using devmem2 while running the VBIOS:<br/>
- <b># devmem2 0xe4361254 w</b>
- </p>
- <p>
- When doing this, it gave back that value. The same trick was used to get backlight controls for T60 (see <a href="#t60_native_notes">#t60_native_notes</a>).
- </p>
-
- <h2>Further notes (as of 6th libreboot release, initial revision)</h2>
- <p>
- Reading <b>0xe4361254</b> (address) in Lenovo BIOS always yields FFFFFFFF, even when writing to it (and writing to it doesn't affect brightness controls).
- 'mtjm' on IRC found that the buttons (Fn keys) control /sys/class/backlight/acpi_video0 which has no affect on 61254 (BLC_PWM_CTL). He says
- intel_backlight has different values and uses the register. devmem2 works, needs checking <b>lspci -vv</b> for where the memory is mapped,
- which is different than on coreboot; mtjm found that it was 0xec061254 on his machine (X60 Tablet), and the register value is different too.
- <b>This is relevant, because we still don't know how backlight controls are actually handled. We got it working by accident. We need to know more.</b>.
- </p>
- <p>
- Intel-gpu-tools may prove useful for further debugging: <a href="http://cgit.freedesktop.org/xorg/app/intel-gpu-tools/">http://cgit.freedesktop.org/xorg/app/intel-gpu-tools/</a>
- </p>
- <p>
- mtjm says 0xe4300000 is an MMIO region of the gpu (lspci -vv shows it), 0x61254 (BLC_PWM_CTL) is a documented register. Searching the kernel driver for backlight
- shows that in intel_panel.c this register is used (there is an XXX comment about finding the right value, where recent kernels get it from.
- </p>
- <p>
- What we want to do is calculate a good value, instead of setting it in devicetree.cb. mtjm says about backlight physics:
- it has a light source , uses pulse width modulation (PWM) to turn it on/off, dimming is done by spending less time on.
- <b>Note: this may not be correct; he says his understanding is based on how the Lenote yeeloong works</b>.
- </p>
- <p>
- mtjm goes on to say, that the register specifies the frequency used for PWM in its depending on the GPU core frequency, so it
- might be possible to calculate it without hardcoded laptop-specific values. Therefore, I am supposed to find out the 'display core frequency'
- (mtjm says there might be a register for it; also, it might be in 5320 or the replay code) and the PWM modulation frequency.
- https://en.wikipedia.org/wiki/Backlight#Flicker_due_to_backlight_dimming
- </p>
- <p>
- phcoder (Vladimir Serbinenko) who is author of 5320 (review.coreboot.org) talks about 'duty cycle limit' and 'flickering frequency'.
- </p>
-
- <p><a href="#pagetop">Back to top of page</a></p>
-
-<hr/>
-
- <h1 id="t60_native_notes">T60 native graphics initialization (with backlight controls)</h1>
- <p>
- <b>The fix below was done on an earlier version of 5345 (review.coreboot.org) cherry-picked on top of 5320 checkout, but also works on the current version.</b>
- </p>
- <p>
- This change is also included by default in libreboot 6th release.
- </p>
- <p>
- <b>Add backlight controls:</b> in <i>src/mainboard/lenovo/t60/devicetree.cb</i>, change <b>gpu_backlight</b> to <b>0x58BF58BE</b>
- </p>
-
- <p><a href="#pagetop">Back to top of page</a></p>
-
-<hr/>
-
- <p>
- Copyright &copy; 2014 Francis Rowe, All Rights Reserved.<br/>
- See <a href="license.html">license.html</a> for license conditions.
- </p>
-
-</body>
-</html>
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