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Thu, 08 Feb 2024 01:12:31 +0000 (GMT) Received: from nalasex01b.na.qualcomm.com (nalasex01b.na.qualcomm.com [10.47.209.197]) by NALASPPMTA03.qualcomm.com (8.17.1.5/8.17.1.5) with ESMTPS id 4181C2L5014533 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 8 Feb 2024 01:12:02 GMT Received: from [10.110.36.76] (10.80.80.8) by nalasex01b.na.qualcomm.com (10.47.209.197) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1118.40; Wed, 7 Feb 2024 17:12:01 -0800 Message-ID: <0cb39613-ec01-50aa-807f-b537f201dac0@quicinc.com> Date: Wed, 7 Feb 2024 17:12:00 -0800 Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH v13 35/53] ALSA: usb-audio: Prevent starting of audio stream if in use Content-Language: en-US From: Wesley Cheng To: Takashi Iwai CC: , , , , , , , , , , , , , , , , , , , , , References: <20240203023645.31105-1-quic_wcheng@quicinc.com> <20240203023645.31105-36-quic_wcheng@quicinc.com> <87y1bxvj0o.wl-tiwai@suse.de> <877cjg7o0k.wl-tiwai@suse.de> <810161b3-4d98-755f-163f-fdfc9fe37063@quicinc.com> In-Reply-To: <810161b3-4d98-755f-163f-fdfc9fe37063@quicinc.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: nasanex01a.na.qualcomm.com (10.52.223.231) To nalasex01b.na.qualcomm.com (10.47.209.197) X-QCInternal: smtphost X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=5800 signatures=585085 X-Proofpoint-ORIG-GUID: 1qFXwgtLZiHWDEqASuuYCj_Qb9m6oVU8 X-Proofpoint-GUID: 1qFXwgtLZiHWDEqASuuYCj_Qb9m6oVU8 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.272,Aquarius:18.0.1011,Hydra:6.0.619,FMLib:17.11.176.26 definitions=2024-02-07_10,2024-02-07_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 suspectscore=0 spamscore=0 malwarescore=0 phishscore=0 mlxscore=0 priorityscore=1501 mlxlogscore=919 clxscore=1015 lowpriorityscore=0 bulkscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2401310000 definitions=main-2402080004 Hi Takashi, On 2/7/2024 4:02 PM, Wesley Cheng wrote: > Hi Takashi, > > On 2/6/2024 11:05 PM, Takashi Iwai wrote: >> On Wed, 07 Feb 2024 01:08:00 +0100, >> Wesley Cheng wrote: >>> >>> Hi Takashi, >>> >>> On 2/6/2024 5:07 AM, Takashi Iwai wrote: >>>> On Sat, 03 Feb 2024 03:36:27 +0100, >>>> Wesley Cheng wrote: >>>>> >>>>> With USB audio offloading, an audio session is started from the ASoC >>>>> platform sound card and PCM devices.  Likewise, the USB SND path is >>>>> still >>>>> readily available for use, in case the non-offload path is >>>>> desired.  In >>>>> order to prevent the two entities from attempting to use the USB bus, >>>>> introduce a flag that determines when either paths are in use. >>>>> >>>>> If a PCM device is already in use, the check will return an error to >>>>> userspace notifying that the stream is currently busy.  This >>>>> ensures that >>>>> only one path is using the USB substream. >>>>> >>>>> Signed-off-by: Wesley Cheng >>>> >>>> Hm, I'm not sure whether it's safe to hold chip->mutex there for the >>>> long code path.  It even kicks off the auto-resume, which may call >>>> various functions at resuming, and some of them may re-hold >>>> chip->mutex. >>>> >>> >>> That's a good point. >>> >>>> If it's only about the open flag, protect only the flag access with >>>> the mutex, not covering the all open function.  At least the re-entry >>>> can be avoided by that. >>>> >>> >>> Sure, let me re-order the check/assignment and the mutex locking. >>> Since this is now checked here in USB PCM and the QC offload driver, >>> we want to make sure that if there was some application attempting to >>> open both at the same time, we prevent any possible races. >>> >>> I think the best way to address this would be something like: >>> >>> static int snd_usb_pcm_open(struct snd_pcm_substream *substream) >>> { >>> ... >>>     mutex_lock(&chip->mutex); >>>     if (subs->opened) { >>>         mutex_unlock(&chip->mutex); >>>         return -EBUSY; >>>     } >>>     subs->opened = 1; >>>     mutex_unlock(&chip->mutex); >>> >>> //Execute bulk of PCM open routine >>> ... >>>     return 0; >>> >>> // If any errors are seen, unwind >>> err_resume: >>>     snd_usb_autosuspend(subs->stream->chip); >>> err_open: >>>     mutex_lock(&chip->mutex); >>>     subs->opened = 0; >>>     mutex_unlock(&chip->mutex); >>> >>>     return ret; >>> } >>> >>> Set the opened flag first, so that if QC offload checks it, it can >>> exit early and vice versa.  Otherwise, if we set the opened flag at >>> the same position as the previous patch, we may be calling the other >>> routines in parallel to the QC offload enable stream routine.  The >>> only thing with this patch is that we'd need some error handling >>> unwinding. >> >> The above is what I had in mind. >> >> But, thinking on this again, you might be able to get the same result >> by using the ALSA PCM core substream open_mutex and hw_opened flag. >> This is already held and set at snd_pcm_core() (the hw_opened flag is >> set after open callback, though).  The offload driver can use those >> instead of the own lock and flag, too, although it's not really >> well-mannered behavior (hence you need proper comments). >> > > I think I had looked into this as well previously, and it was difficult > to achieve, because from the USB offloading perspective, we don't ever > call: snd_usb_pcm_open() > > This is actually where we populate the pcm_substream parameter within > struct snd_usb_substream based on when userspace opens the USB SND PCM > device (which is not the case for offloading).  So the offload driver > doesn't have a way to fetch the struct snd_pcm that is allocated to the > PCM device created by the USB SND card. > Sorry, took a look at it again, and found a way. Although not pretty, we can access it using: subs->stream->pcm->streams[direction].substream->hw_opened Thanks Wesley Cheng