Re: Slow I/O on USB media

[Andrea Vai <andrea.vai@unipv.it>]

Hi,
Il giorno mer, 05/06/2019 alle 10.26 -0400, Alan Stern ha scritto:
> On Wed, 5 Jun 2019, Andrea Vai wrote:
> 
> > Hi,
> > Il giorno mar, 04/06/2019 alle 07.43 +0200, Greg KH ha scritto:
> > > On Mon, Jun 03, 2019 at 01:13:48PM +0200, Andrea Vai wrote:
> > > > Il giorno gio, 30/05/2019 alle 06.25 -0700, Greg KH ha
> scritto:
> > > > > [...]
> > > > Hi,
> > > > 
> > > > > Any chance you can use 'git bisect' to find the offending
> > > commit?
> > > > Yes, I am doing it as I managed to build the kernel from
> source
> > > 
> > > Great!  What did you find?
> > 
> > # first bad commit: [534903d60376b4989b76ec445630aa10f2bc3043]
> > drm/atomic: Use explicit old crtc state in
> > drm_atomic_add_affected_planes()
> > 
> > By the way, as I am not expert, is there a way to double-check
> that I
> > bisected correctly? (such as, e.g., test with the version before
> this
> > one, and then with this commit applied?)
> 
> That is exactly the way to do it: Build a kernel from that commit
> and 
> see that it fails, then revert the commit and see that the
> resulting 
> kernel succeeds.
> 
> (Note: The notion of "version before" doesn't have a firm meaning
> in 
> the kernel, because some commits have multiple parents.  The best
> way 
> to see if a single commit caused a change is to do what I said
> above: 
> revert the commit and see what happens.)
ok, thank you for pointing it out. So, my question is: how to revert a
commit? (sorry, I prefer to ask you because I am afraid I could do
something wrong, and don't trust too much myself and what I pick up
searching on the web. In the special case, I found "git revert", but
for example how could I revert back a "reversion"? :-/ (I know I miss
the basis, I never worked with git, so sorry for the stupid
question)).

> 
> Incidentally, it seems very unlikely that a commit for the drm 
> subsystem would have any effect on the behavior of a USB storage 
> device.

well, I had the same doubt and that's the reason I was trying to do
the check: I'm afraid I have done something wrong or made a mess with
the bisect process.

Thank you,
Andrea