Re: [PATCH 3/6] fms: Add TOC/PMA/ATIP DVD-ROM capabilities

On Sun, Jun 27, 2021 at 09:45:07PM +0300, i.kononenko wrote:

On 27.06.2021 17:29, Alan Stern wrote:

quoted

Is any of this really needed?  What usage scenarios require
f_mass_storage to emulate a DVD-ROM that couldn't use f_tcm instead?

I can't see any impediments to supplement the already existing 
implementation of MMC-(2/3) specification of multimedia devices to 
represent the DVD/BD features. If the kernel presents the CD-ROM SCSI 
commands, why the mass_storage:usb-gadget-function still doesn't include
that for DVD/BD?

Many modern embedded systems (e.g., BMC, OpenBMC) implements their 
required features, e.g., Virtual Media Device, which is based on the 
usb:gadget:mass-storage. 
The purpose of that features is extensive, and their use the mass-storage
not only as a cdrom-device.

The required features of such systems might expect image back-end files
that size is significant than 2.1Gb, but such medium is not the CD-ROM 
device. USB-gadget consumers can incorrectly interpret such device by 
loading the wrong driver. I believe that should be the DVD-medium device,
at least. 

You should include this information in the patch description, so that 
people will understand why you wrote the patch.

Additionally, please note the current patch also fixes the incorrect 
implementation of retrieving TOC/PMA/ATIP data, which is required for the 
CD-ROM. One system might correct works with retrieving first with the 
last session together, but for some systems, e.g., OS ESXi, OS Windows, 
should retrieving first and last border sessions in separate SCSI-request. 

What's wrong with the existing implementation?  Are you talking about 
the do_read_toc function?  The driver only supports one session in any 
case.

In general, fixes to existing code and additions of new code should go 
in separate patches.

Alan Stern