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[78.88.45.245]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6507b8c4479sm9806221a12.1.2026.01.09.01.06.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Jan 2026 01:06:32 -0800 (PST) Message-ID: Date: Fri, 9 Jan 2026 10:06:29 +0100 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] drm/msm/dp: Avoid division by zero in msm_dp_ctrl_config_msa() To: Nathan Chancellor , Rob Clark , Dmitry Baryshkov , Abhinav Kumar , Jessica Zhang , Sean Paul , Marijn Suijten Cc: linux-arm-msm@vger.kernel.org, dri-devel@lists.freedesktop.org, freedreno@lists.freedesktop.org, llvm@lists.linux.dev, patches@lists.linux.dev, kernel test robot References: <20260108-drm-msm-dp_ctrl-avoid-zero-div-v1-1-6a8debcb3033@kernel.org> Content-Language: en-US From: Konrad Dybcio In-Reply-To: <20260108-drm-msm-dp_ctrl-avoid-zero-div-v1-1-6a8debcb3033@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Authority-Analysis: v=2.4 cv=Ue1ciaSN c=1 sm=1 tr=0 ts=6960c519 cx=c_pps a=qKBjSQ1v91RyAK45QCPf5w==:117 a=FpWmc02/iXfjRdCD7H54yg==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=s4-Qcg_JpJYA:10 a=VkNPw1HP01LnGYTKEx00:22 a=NEAV23lmAAAA:8 a=VwQbUJbxAAAA:8 a=QyXUC8HyAAAA:8 a=xIqhFIF2P0JOsuq1WLcA:9 a=QEXdDO2ut3YA:10 a=NFOGd7dJGGMPyQGDc5-O:22 X-Proofpoint-ORIG-GUID: nOnvq5vsmeWFHSYt6oDIZqhLkzyzHrbT X-Proofpoint-GUID: nOnvq5vsmeWFHSYt6oDIZqhLkzyzHrbT X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTA5MDA2NCBTYWx0ZWRfX6J+spLrx2GtT dvbrBnmxXbi1cW5rQ2cMuthM5x61FdMoNXnrw9nu2ORUZiJWCgs80qeGDnl3qGAdBmYNDIjmcqj Z2AMwTzRc9Ox7a0FqF6iQSvnaqrC/e4vydgXyku09TcePfO0UE/RP6Uau+jaapAh86akYQXQyrY pkr2mbuvrMB0MTf1B/ke8bGMB8+mGUgUMkhj7HJBoGI0Es7qwLpPDBbiMp6A+sJPBAErJH3nm1V q+rThuSHkQfm5k/P5IBkW8RNFgMQxnjxxwxOLaDqqZDEeVQ9X9lN4kq1Hw/HGFl2aSHOJY+NZwN Dbc04jL0HWCyIYCqclq2Kv+ExlgLhGW0haqJ0q8FWl+AzzjupJBKWLVOaVdGDxiShp9FUIEp+ET +wq8dl21EplEa1ZMqVsNhzRncv5KgzHIwNE3QHo4CQ4DGuhS+XvePt2v0dai0Oeciw8YYOsHlUE PBC/OjfYOiAcR0cXELQ== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2026-01-09_02,2026-01-08_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 lowpriorityscore=0 clxscore=1011 priorityscore=1501 bulkscore=0 suspectscore=0 phishscore=0 spamscore=0 malwarescore=0 adultscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2512120000 definitions=main-2601090064 On 1/9/26 12:29 AM, Nathan Chancellor wrote: > An (admittedly problematic) optimization change in LLVM 20 [1] turns > known division by zero into the equivalent of __builtin_unreachable(), > which invokes undefined behavior if it is encountered in a control flow > graph, destroying code generation. When compile testing for x86_64, > objtool flags an instance of this optimization triggering in > msm_dp_ctrl_config_msa(), inlined into msm_dp_ctrl_on_stream(): > > drivers/gpu/drm/msm/msm.o: warning: objtool: msm_dp_ctrl_on_stream(): unexpected end of section .text.msm_dp_ctrl_on_stream > > The zero division happens if the else branch in the first if statement > in msm_dp_ctrl_config_msa() is taken because pixel_div is initialized to > zero and it is not possible for LLVM to eliminate the else branch since > rate is still not known after inlining into msm_dp_ctrl_on_stream(). > Change pixel_div to one to make the division well defined in the > presence of an unsupported rate, relying on the DRM_ERROR print to > indicate the error to the user. > > Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support") > Link: https://github.com/llvm/llvm-project/commit/37932643abab699e8bb1def08b7eb4eae7ff1448 [1] > Reported-by: kernel test robot > Closes: https://lore.kernel.org/oe-kbuild-all/202601081959.9UVJEOfP-lkp@intel.com/ > Signed-off-by: Nathan Chancellor > --- > drivers/gpu/drm/msm/dp/dp_ctrl.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c > index cbcc7c2f0ffc..e4731c059ed8 100644 > --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c > +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c > @@ -2395,7 +2395,7 @@ static void msm_dp_ctrl_config_msa(struct msm_dp_ctrl_private *ctrl, > bool is_ycbcr_420) > { > u32 pixel_m, pixel_n; > - u32 mvid, nvid, pixel_div = 0, dispcc_input_rate; > + u32 mvid, nvid, pixel_div = 1, dispcc_input_rate; > u32 const nvid_fixed = DP_LINK_CONSTANT_N_VALUE; > u32 const link_rate_hbr2 = 540000; > u32 const link_rate_hbr3 = 810000; > > --- > base-commit: 66691e272e40c91305f1704695e0cb340cd162ff > change-id: 20260108-drm-msm-dp_ctrl-avoid-zero-div-be5dc40cbc18 Dmitry, would it be beneficial to throw an actual error when the rate is is mangled? i.e. diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c index aa2303d0e148..4f710b8e6bc6 100644 --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c @@ -2404,9 +2404,9 @@ static int msm_dp_ctrl_link_retrain(struct msm_dp_ctrl_private *ctrl) return msm_dp_ctrl_setup_main_link(ctrl, &training_step); } -static void msm_dp_ctrl_config_msa(struct msm_dp_ctrl_private *ctrl, - u32 rate, u32 stream_rate_khz, - bool is_ycbcr_420) +static int msm_dp_ctrl_config_msa(struct msm_dp_ctrl_private *ctrl, + u32 rate, u32 stream_rate_khz, + bool is_ycbcr_420) { u32 pixel_m, pixel_n; u32 mvid, nvid, pixel_div = 0, dispcc_input_rate; @@ -2415,14 +2415,21 @@ static void msm_dp_ctrl_config_msa(struct msm_dp_ctrl_private *ctrl, u32 const link_rate_hbr3 = 810000; unsigned long den, num; - if (rate == link_rate_hbr3) + switch (rate) { + case link_rate_hbr3: pixel_div = 6; - else if (rate == 162000 || rate == 270000) - pixel_div = 2; - else if (rate == link_rate_hbr2) + break; + case link_rate_hbr2: pixel_div = 4; - else + break; + case 270000: + case 162000: + pixel_div = 2; + break; + default: DRM_ERROR("Invalid pixel mux divider\n"); + return -EINVAL; + } dispcc_input_rate = (rate * 10) / pixel_div; @@ -2458,6 +2465,8 @@ static void msm_dp_ctrl_config_msa(struct msm_dp_ctrl_private *ctrl, drm_dbg_dp(ctrl->drm_dev, "mvid=0x%x, nvid=0x%x\n", mvid, nvid); msm_dp_write_link(ctrl, REG_DP_SOFTWARE_MVID, mvid); msm_dp_write_link(ctrl, REG_DP_SOFTWARE_NVID, nvid); + + return 0; } int msm_dp_ctrl_on_stream(struct msm_dp_ctrl *msm_dp_ctrl, bool force_link_train) @@ -2525,10 +2534,11 @@ int msm_dp_ctrl_on_stream(struct msm_dp_ctrl *msm_dp_ctrl, bool force_link_train msm_dp_ctrl_configure_source_params(ctrl); - msm_dp_ctrl_config_msa(ctrl, - ctrl->link->link_params.rate, - pixel_rate_orig, - ctrl->panel->msm_dp_mode.out_fmt_is_yuv_420); + ret = msm_dp_ctrl_config_msa(ctrl, ctrl->link->link_params.rate, + pixel_rate_orig, + ctrl->panel->msm_dp_mode.out_fmt_is_yuv_420); + if (ret) + return ret; msm_dp_panel_clear_dsc_dto(ctrl->panel); Konrad