Re: [Qemu-devel] [PATCH] block.c: fix real cdrom detection

[Laurent Vivier]

On 25/06/2015 18:16, Paolo Bonzini wrote:
> 
> 
> On 25/06/2015 18:12, Laurent Vivier wrote:
>>
>>
>> On 25/06/2015 17:48, Paolo Bonzini wrote:
>>>
>>> On 25/06/2015 17:32, Programmingkid wrote:
>>>>> I think we are going to have to agree to disagree. I have never
>>>>> used the /dev/sr(0 | 1) devices and don't see how they would be
>>>>> effected by this patch. Are you trying to say the /dev/sr(0 | 1)
>>>>> devices *should* be handled by this patch?
>>>>
>>>> Thinking about your question some more, I see what you mean. On Linux
>>>> /dev/sr0 refers to the cdrom drive. Also on Linux, the /dev/cdrom
>>>> link refers to the /dev/sr0 device file. So if you just use
>>>> /dev/cdrom, you are good.
>>>
>>> Well, that's not how things work.
>>>
>>> If you do things like that, you end up with a bunch of hacks, not with a
>>> decent piece of software.
>>>
>>> There is support for CD-ROM passthrough on Linux and FreeBSD in
>>> block/raw-posix.c.  Perhaps the FreeBSD support can be extended to OS X
>>> as well.
>>>
>>
>> In fact, programmingkid, you should fix it in hdev_open() where there is
>> already a #if __APPLE__ .
>>
>> Paolo, I agree with you but :
>>
>> hdev_open()
>>
>> #if defined(__linux__)
>>     {
>>         char resolved_path[ MAXPATHLEN ], *temp;
>>
>>         temp = realpath(filename, resolved_path);
>>         if (temp && strstart(temp, "/dev/sg", NULL)) {
>>             bs->sg = 1;
>>         }
>> #endif
>>
>> I'm wondering who had this strange idea... :)
> 
> I was very scared to type "git blame" here. :)  But the question is also

http://geek-and-poke.com/2013/11/24/simply-explained

BTW, it is a legacy from 2006:

19cb373 better support of host drives

coming from MacOS X (again!):

3b0d4f6 OS X: support for the built in CD-ROM drive (Mike Kronenberg)

> where to put the checks.  Putting it at a random place in block.c is not
> a good idea.
> 
> But yes, this is also bad.  It should use stat and check the major/minor
> numbers.

Yes, we should check if major is SCSI_GENERIC_MAJOR (21) (on linux).

We can also try to send an SG command like in cdrom_probe_device().
Something like in scsi_generic_realize():

    rc = blk_ioctl(s->conf.blk, SG_GET_VERSION_NUM, &sg_version);
    if (rc < 0) {
        error_setg(errp, "cannot get SG_IO version number: %s.  "
                         "Is this a SCSI device?",
                         strerror(-rc));
        return;
    }

Laurent