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Message-ID: <5fc5439b-ef21-416a-90fe-07f0d21de87d@suse.de>
Date: Thu, 22 Jan 2026 11:39:45 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Icenowy Zheng <uwu@...nowy.me>, Andrzej Hajda <andrzej.hajda@...el.com>,
Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>,
Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>, David Airlie <airlied@...il.com>,
Simona Vetter <simona@...ll.ch>, Rob Herring <robh@...nel.org>,
Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
<conor+dt@...nel.org>, Drew Fustini <fustini@...nel.org>,
Guo Ren <guoren@...nel.org>, Fu Wei <wefu@...hat.com>
Cc: Philipp Zabel <p.zabel@...gutronix.de>,
Dmitry Baryshkov <lumag@...nel.org>,
Michal Wilczynski <m.wilczynski@...sung.com>,
Luca Ceresoli <luca.ceresoli@...tlin.com>, Han Gao <rabenda.cn@...il.com>,
Yao Zi <ziyao@...root.org>, linux-kernel@...r.kernel.org,
dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
linux-riscv@...ts.infradead.org, Han Gao <gaohan@...as.ac.cn>
Subject: Re: [PATCH v5 3/9] drm: verisilicon: add a driver for Verisilicon
display controllers
Hi
Am 22.01.26 um 10:23 schrieb Icenowy Zheng:
> 在 2026-01-21星期三的 13:56 +0100，Thomas Zimmermann写道：
>
> =============== 8< ===================
>>> +
>>> + dev_info(dev, "DC%x rev %x customer %x\n", dc-
>>>> identity.model,
>>> + dc->identity.revision, dc->identity.customer_id);
>> drm_dbg(). Drivers should be quite by default.
> Well for this kind of identification information I think driver is
> usually not quiet, at least amdgpu (when doing IP discovery), panfrost
> and etnaviv (which shares the same set of identification registers with
> this driver) is reporting it.
These drivers only get away with it because many other drivers keep
quite. Otherwise the kernel log would be filled with init reports. Your
choice, but it's questionable if anyone ever cares except for debugging.
>
>>> +
>>> + if (port_count > dc->identity.display_count) {
>>> + dev_err(dev, "too many downstream ports than HW
>>> capability\n");
>>> + ret = -EINVAL;
>>> + goto err_rst_assert;
>>> + }
>>> +
> =============== 8< ===================
>>> +
>>> + if (!state->visible || !fb) {
>>> + regmap_write(dc->regs, VSDC_FB_CONFIG(output), 0);
>>> + regmap_write(dc->regs, VSDC_FB_CONFIG_EX(output),
>>> 0);
>>> + goto commit;
>>> + } else {
>>> + regmap_set_bits(dc->regs,
>>> VSDC_FB_CONFIG_EX(output),
>>> + VSDC_FB_CONFIG_EX_FB_EN);
>>> + }
>> Since you handle the plane on/plane off cases here, I'd advise to
>> implement atomic_enable and atomic_disable for the plane, if the
>> hardware allows it. (There is hardware were the current pattern makes
>> sense though.)
> If atomic_*able is going to be implemented, should atomic_update() keep
> the plane on/off code?
In this case, atomic_update should only perform an update of the plane's
graphics buffer (scanout address, color format). The logic then is
enable and mode setting:
atomic_update, plus
atomic_enable
disable:
atomic_disable only
page flips:
atomic_update only
See
https://elixir.bootlin.com/linux/v6.18.6/source/drivers/gpu/drm/drm_atomic_helper.c#L3022
That's usually cleaner. But there's hardware where update/enable/disable
is connected in such a way that a single atomic_update is better.
>
> BTW it seems that DC8200 has the shadow register capability that could
> be manually commited but the older DC8000 has no (the
> VSDC_FB_CONFIG_EX_COMMIT bit written below is a new thing) -- the
> VSDC_FB_CONFIG register on DC8000 has a write-only bit that when
> written with 0 the display is put to reset and when written with 1 the
> display will start to run. This pattern seems to be not able to keep
> the enabled state while programming (at least part of) new plane
> configuration, see [1].
I cannot comment on the hardware. But on the DRM side, we always disable
the pipeline for we program a new display mode; and then do an enable
step to program the new mode. This happens on the CRTC, but it also
affects the CRTC's planes accordingly. For pageflips, we only run the
plane's atomic_update.
If you you see a page flip, but need a full mode-setting operation, you
can manually trigger it by setting drm_crtc_state.mode_changed from the
plane's atomic_check. Here's an example:
https://elixir.bootlin.com/linux/v6.18.6/source/drivers/gpu/drm/mgag200/mgag200_mode.c#L503
If the DC8000 and DC8200 behave sufficiently differently, my advise is
to write multiple _funcs structs with custom helpers and pick the
correct one when you initialize the modesetting pipeline.
Best regards
Thomas
>
> [1]
> https://github.com/milkv-megrez/rockos-u-boot/blob/c9221cf2fa77d39c0b241ab4b030c708e7ebe279/drivers/video/eswin/eswin_dc_reg.h#L3579
>
>>> +
>>> + drm_format_to_vs_format(state->fb->format->format, &fmt);
>>> +
>>> + regmap_update_bits(dc->regs, VSDC_FB_CONFIG(output),
>>> + VSDC_FB_CONFIG_FMT_MASK,
>>> + VSDC_FB_CONFIG_FMT(fmt.color));
>>> + regmap_update_bits(dc->regs, VSDC_FB_CONFIG(output),
>>> + VSDC_FB_CONFIG_SWIZZLE_MASK,
>>> + VSDC_FB_CONFIG_SWIZZLE(fmt.swizzle));
>>> + regmap_assign_bits(dc->regs, VSDC_FB_CONFIG(output),
>>> + VSDC_FB_CONFIG_UV_SWIZZLE_EN,
>>> fmt.uv_swizzle);
>>> +
>>> + /* Get the physical address of the buffer in memory */
>>> + gem = drm_fb_dma_get_gem_obj(fb, 0);
>>> +
>>> + /* Compute the start of the displayed memory */
>>> + bpp = fb->format->cpp[0];
>>> + dma_addr = gem->dma_addr + fb->offsets[0];
>>> +
>>> + /* Fixup framebuffer address for src coordinates */
>>> + dma_addr += (state->src.x1 >> 16) * bpp;
>> bpp is deprecated and should be avoided in new code. You can compute
>> the
>> offset with drm_format_min_pitch():
>>
>> drm_format_min_pitch(fb->format, 0, state->src.x1 >> 16 )
>>
>> Best regards
>> Thomas
>>
>>> + dma_addr += (state->src.y1 >> 16) * fb->pitches[0];
>>> +
>>> + regmap_write(dc->regs, VSDC_FB_ADDRESS(output),
>>> + lower_32_bits(dma_addr));
>>> + regmap_write(dc->regs, VSDC_FB_STRIDE(output),
>>> + fb->pitches[0]);
>>> +
>>> + regmap_write(dc->regs, VSDC_FB_TOP_LEFT(output),
>>> + VSDC_MAKE_PLANE_POS(state->crtc_x, state-
>>>> crtc_y));
>>> + regmap_write(dc->regs, VSDC_FB_BOTTOM_RIGHT(output),
>>> + VSDC_MAKE_PLANE_POS(state->crtc_x + state-
>>>> crtc_w,
>>> + state->crtc_y + state-
>>>> crtc_h));
>>> + regmap_write(dc->regs, VSDC_FB_SIZE(output),
>>> + VSDC_MAKE_PLANE_SIZE(state->crtc_w, state-
>>>> crtc_h));
>>> +
>>> + regmap_write(dc->regs, VSDC_FB_BLEND_CONFIG(output),
>>> + VSDC_FB_BLEND_CONFIG_BLEND_DISABLE);
>>> +commit:
>>> + regmap_set_bits(dc->regs, VSDC_FB_CONFIG_EX(output),
>>> + VSDC_FB_CONFIG_EX_COMMIT);
>>> +}
>>> +
> =============== 8< ===================
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)
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