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Fri, 18 Mar 2022 02:58:28 +0000 Received: from BY5PR12MB4130.namprd12.prod.outlook.com ([fe80::b4d2:1b8:7a7b:fdcb]) by BY5PR12MB4130.namprd12.prod.outlook.com ([fe80::b4d2:1b8:7a7b:fdcb%7]) with mapi id 15.20.5081.018; Fri, 18 Mar 2022 02:58:28 +0000 Message-ID: Date: Thu, 17 Mar 2022 19:58:25 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0 Subject: Re: [PATCH v2] mm/hmm/test: simplify hmm test code: use miscdevice instead of char dev Content-Language: en-US To: =?UTF-8?Q?Mika_Penttil=c3=a4?= , Jason Gunthorpe Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, apopple@nvidia.com, rcampbell@nvidia.com, vbabka@suse.cz References: <20220311033050.22724-1-mpenttil@redhat.com> <20220314182439.GB64706@ziepe.ca> <20220315183922.GC64706@ziepe.ca> <8e836d75-97b0-d301-4d6a-92025e91cad5@redhat.com> <0f53c5ff-1711-861d-9a12-0634d4ab9fca@redhat.com> <20220317141539.GD64706@ziepe.ca> <10fb1261-f22f-8e21-4056-166c283e26f2@redhat.com> From: John Hubbard In-Reply-To: <10fb1261-f22f-8e21-4056-166c283e26f2@redhat.com> Content-Type: text/plain; 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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=IrlhYwBS; dmarc=pass (policy=reject) header.from=nvidia.com; spf=none (imf29.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.236.59) smtp.mailfrom=jhubbard@nvidia.com X-Stat-Signature: qusnxpfxkac1wppze77eduqt37brfff4 X-Rspamd-Server: rspam04 X-HE-Tag: 1647572309-330449 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000039, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 3/17/22 19:34, Mika Penttil=C3=A4 wrote: > On 17.3.2022 16.15, Jason Gunthorpe wrote: >> On Thu, Mar 17, 2022 at 08:58:52AM +0200, Mika Penttil=C3=A4 wrote: >>> @@ -1225,7 +1232,11 @@ static int dmirror_device_init(struct dmirror_= device >>> *mdevice, int id) >>> >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 cdev_init(&mdevice->= cdevice, &dmirror_fops); >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 mdevice->cdevice.own= er =3D THIS_MODULE; >>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ret =3D cdev_add(&mdevice->cdev= ice, dev, 1); >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 device_initialize(&mdevice->dev= ice); >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 dev_set_name(&mdevice->device, = "%s", dmirror_device_names[id]); >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 mdevice->device.devt =3D dev; >>> + >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ret =3D cdev_device_add(&mdevic= e->cdevice, &mdevice->device); >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (ret) >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 return ret; >> >> Right, miscdev isn't that helpful in the end.. >> >> Jason >> >=20 > To wrap up, I could send a v3 formal patch with either this cdev way or= the miscdev way.. Both=20 > eliminate the user space /proc/devices parsing and mknod'ing. Jason bro= ught up concerns using=20 > miscdevice like this, although for this case it works correctly. miscde= vice also provides a little=20 > more cleanup and simpler code, but the difference is not huge. So what = do people prefer? >=20 No strong preference from me either way, but I would like to bottom on the potential problems with using misdevice. The fact that miscdevice is misused in other drivers does not seem like a reason that it must be not used here...Jason, after Mika's explanation, what's your latest assessment? In other words, given that the usage here is correct, is there some reason that miscdevice is still a poor design fit? Is cdev_device_add() a better choice here, for design reasons? Also, is there any change that could or should be made to miscdevice, that you have in mind? thanks, --=20 John Hubbard NVIDIA