Re: [PATCH v7 1/7] drm/lsdc: add drm driver for loongson display controller

[Sui Jingfeng]

On 2022/2/14 18:10, Maxime Ripard wrote:
> On Sun, Feb 13, 2022 at 10:16:43PM +0800, Sui Jingfeng wrote:
> > From: suijingfeng <suijingfeng@xxxxxxxxxxx>
> >
> > There is a display controller in loongson's LS2K1000 SoC and LS7A1000
> > bridge chip, the DC is a PCI device in those chips. It has two display
> > pipes but with only one hardware cursor. Each way has a DVO interface
> > which provide RGB888 signals, vertical & horizontal synchronisations,
> > data enable and the pixel clock. Each CRTC is able to scanout from
> > 1920x1080 resolution at 60Hz. The maxmium resolution is 2048x2048
> > according to the hardware spec.
> >
> > Loongson display controllers are simple which require scanout buffers
> > to be physically contiguous. LS2K1000 is a SOC, Only system memory is
> > available. Therefore CMA helper based driver is intended to be use,
> > although it is possible to use VRAM helper based solution by carving
> > out part of system memory as VRAM.
> >
> > On LS7A1000/LS7A2000 bridge chip, the DC is equipped with a dedicated
> > video memory which is typically 64MB or more. In this case, VRAM helper
> > based solution which scanout from local VRAM is reconmended to use.
> > It is reliable to use for massive production, but CMA based helper
> > solution is still usable on ls7a1000 and ls7a2000, at the price of
> > the CRTC must access the FB in RAM through the PCIe bus and HT3.0 bus.
> > This causes continuous traffic on the bus regardless of whether the FB
> > image is updating or not. Sadly, it suffer from screen flickering under
> > RAM pressure on LS7A1000. Luckily, It show extremely good condition on
> > LS7A2000 even under stressapptest, Maybe the hardware engineer resolve
> > this issue. Integrating two distict helpers based driver into one piece
> > allow code sharing.
> >
> > We have also implemented demage update on top of CMA helper which copy
> > the demaged region from the shadow framebuffer in system RAM to the real
> > framebuffer in VRAM manually. This is intend to overcome the screen
> > flicking issue on LS7A1000, but the performance is not good.
> > Using "lsdc.dirty_update=1" in the kernel commmand line if you would like
> > to have a try.
> >
> > For LS7A1000, there are 4 dedicated GPIOs whose control register is
> > located at the DC register space, They are used to emulate two way i2c.
> > One for DVO0, another for DVO1. This is the reason why this driver is
> > not switch to drm bridge framework yet. LS2K1000 and LS2K0500 SoC don't
> > have such GPIO hardwared, they grab i2c adapter from other module,
> > either general purpose GPIO emulated i2c or hardware i2c adapter.
> > Drm bridge and drm panel driver for the external encoder is suitable for
> > those SoC. We have already implemented this on our downstream 4.19.190
> > kernel. But due to the GPIO, PWM and I2C device driver support for
> > LS2K1000 is not upstreamed yet, this driver still can be use to bring
> > the graphic environment up by providing display timings or similar things
> > in the device tree.
> >
> > The DC in LS7A1000 has only one hardware cursor, we simply let the two
> > CRTC share it. The DC in LS7A2000 have two cursor, two built-in hdmi
> > encoder and one transparent vga encoder and more, surport for LS7A2000
> > is on the way. In short, we have built-in gpio emulated i2c support,
> > we also have hardware cursor support. LS7A2000 The kind of tiny drivers
> > in drm/tiny is not suitable for us.
> >
> >      +------+            +-----------------------------------+
> >      | DDR4 |            |  +-------------------+            |
> >      +------+            |  | PCIe Root complex |   LS7A1000 |
> >         || MC0           |  +--++---------++----+            |
> >    +----------+  HT 3.0  |     ||         ||                 |
> >    | LS3A4000 |<-------->| +---++---+  +--++--+    +---------+   +------+
> >    |   CPU    |<-------->| | GC1000 |  | LSDC |<-->| DDR3 MC |<->| VRAM |
> >    +----------+          | +--------+  +-+--+-+    +---------+   +------+
> >         || MC1           +---------------|--|----------------+
> >      +------+                            |  |
> >      | DDR4 |          +-------+   DVO0  |  |  DVO1   +------+
> >      +------+   VGA <--|ADV7125|<--------+  +-------->|TFP410|--> DVI/HDMI
> >                        +-------+                      +------+
> >
> > The above picture give a simple usage of LS7A1000, note that the encoder
> > is not necessary adv7125 or tfp410, it is a choice of the downstream board
> > manufacturer. Other candicate encoders can be ch7034b, sil9022 and ite66121
> > lt8618 etc. Besides, the DC in both ls2k1000 and ls7k1000 has the same of
> > PCI vendor id and pci device id. Both is 0x0014:0x7a06, the reverison id
> > is also same. This is the firmware engineer's mistake, but such firmware
> > and various boards ship with such firmware already released. We choose to
> > deduce the chip's identification from information provided by device tree.
> > For lsdc, there is only a 1:1 mapping of encoders and connectors.
> >
> > v2: fixup warnings reported by kernel test robot
> >
> > v3: fix more grammar mistakes in Kconfig reported by Randy Dunlap and give
> >      more details about lsdc.
> >
> > v4:
> >     1) Add dts required and explain why device tree is required.
> >     2) Give more description about lsdc and vram helper base driver.
> >     3) Fix warnings reported by kernel test robot.
> >     4) Introduce stride_alignment member into struct lsdc_chip_desc, the
> >        stride alignment is 256 bytes for ls7a1000, ls2k1000 and ls2k0500.
> >        But ls7a2000 improve it to 32 bytes, for extend the support for the
> >        device on coming
> >
> > v5:
> >     1) using writel and readl replace writeq and readq, to fix kernel test
> >        robot report build error on other archtecture
> >     2) set default fb format to XRGB8888 at crtc reset time.
> >     3) fix typos.
> >
> > v6:
> >     1) Explain why we are not switch to drm dridge subsystem on ls2k1000.
> >     2) Explain why tiny drm driver is not suitable for us.
> >     3) Give a short description of the trival dirty uppdate implement based
> >        on CMA helper.
> >     4) code clean up
> >
> > v7:
> >     1) Remove select I2C_GPIO and I2C_LS2X in Kconfig, it is not ready now
> >     2) Licensing issues are fixed suggested by Krzysztof Kozlowski.
> >     3) lsdc_pixpll_print() is removed, part of it move to debugfs.
> >     4) Set prefer_shadow to true if vram based driver is in using.
> >     5) Replace double blank lines with single line in all files
> >     6) Verbose cmd line parameter is replaced with drm_dbg()
> >     7) All warnnings reported by ./scripts/checkpatch.pl --strict are fixed
> >     8) Get edid from dtb support is removed as suggested by Maxime Ripard
> >     9) Fix typos and various improvement
> you *need* to have a DT bindings description

Ok, I know that, it takes some time to document it though.