linux-kernel - Re: [PATCH] drm/bridge/synopsys: avoid field overflow warning

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Message-Id: <220ed0fb-1ccb-4371-9b5a-a99dfdc84984@app.fastmail.com>
Date: Thu, 10 Apr 2025 14:36:45 +0200
From: "Arnd Bergmann" <arnd@...nel.org>
To: "Arnd Bergmann" <arnd@...db.de>,
 "Andrzej Hajda" <andrzej.hajda@...el.com>,
 "Neil Armstrong" <neil.armstrong@...aro.org>,
 "Robert Foss" <rfoss@...nel.org>,
 "Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>,
 "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "Dave Airlie" <airlied@...il.com>,
 "Simona Vetter" <simona@...ll.ch>, "Nathan Chancellor" <nathan@...nel.org>,
 "Heiko Stuebner" <heiko.stuebner@...rry.de>,
 "Andy Yan" <andy.yan@...k-chips.com>
Cc: "laurent.pinchart" <Laurent.pinchart@...asonboard.com>,
 "Jonas Karlman" <jonas@...boo.se>,
 "Jernej Skrabec" <jernej.skrabec@...il.com>,
 "Nick Desaulniers" <nick.desaulniers+lkml@...il.com>,
 "Bill Wendling" <morbo@...gle.com>, "Justin Stitt" <justinstitt@...gle.com>,
 "Dmitry Baryshkov" <lumag@...nel.org>,
 "Doug Anderson" <dianders@...omium.org>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] drm/bridge/synopsys: avoid field overflow warning

On Wed, Apr 9, 2025, at 09:07, Arnd Bergmann wrote:
> On Tue, Apr 8, 2025, at 19:51, Arnd Bergmann wrote:
>> From: Arnd Bergmann <arnd@...db.de>
>>
>> clang-16 and earlier complain about what it thinks might be an out of
>> range number:
>>
>> drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi2.c:348:8: error: call to 
>> __compiletime_assert_579 declared with 'error' attribute: FIELD_PREP: 
>> value too large for the field
>>                      PHY_SYS_RATIO(tmp));
>>                      ^
>> drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi2.c:90:27: note: expanded 
>> from macro 'PHY_SYS_RATIO'
>>  #define PHY_SYS_RATIO(x)                FIELD_PREP(GENMASK(16, 0), x)
>>
>
> I still see the same build failure in some other configurations even
> with this patch. Please disregard this version, I'll try to come
> up with a better one.

I couldn't come up with anything that actually worked, other than
the hack below, which just works around the compiletime error
in FIELD_PREP(), but doesn't look like a proper fix.

If anyone else has any ideas, I can test their patch.

       Arnd

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi2.c b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi2.c
index c76f5f2e74d1..8ba528462ede 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi2.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi2.c
@@ -344,7 +344,7 @@ static void dw_mipi_dsi2_phy_ratio_cfg(struct dw_mipi_dsi2 *dsi2)
         */
        tmp = DIV_ROUND_CLOSEST_ULL(phy_hsclk << 16, sys_clk);
        regmap_write(dsi2->regmap, DSI2_PHY_SYS_RATIO_MAN_CFG,
-                    PHY_SYS_RATIO(tmp));
+                    PHY_SYS_RATIO(tmp & GENMASK(16, 0)));
 }
 
 static void dw_mipi_dsi2_lp2hs_or_hs2lp_cfg(struct dw_mipi_dsi2 *dsi2)