[U-Boot] [PATCH 01/11] DM: add block device core

[Marek Vasut]

Dear Pavel Herrmann,

> On Saturday 22 of September 2012 02:09:15 Marek Vasut wrote:
> > Dear Pavel Herrmann,
> > 
> > [...]
> > 
> > > > > one or none - requests on USB flashes should not pass through
> > > > > block_controller_driver.
> > > > 
> > > > Uh, what do they pass into then ?
> > > 
> > > their parent (an USB hub)
> > 
> > block_device instance (aka. partition/disk) directly connected to USB hub
> > instance does not seem right.
> 
> why?

It doesn't make sense ... you need some kind of interim controller (like the 
chip between the USB and NAND in the thumbdrive.

> > > > > every child of block_controller should be a block_device (not
> > > > > necessarily the other way around
> > > > 
> > > > I doubt it's even possible to be the other way around.
> > > > 
> > > > > ), so there is no way you pass more instances
> > > > > block_controller on your way up.
> > > > 
> > > > Ok, let me explain again. Let's look at the USB case to make it more
> > > > real-world- ish. Imagine you have a thumb drive with 2 partitions.
> > > > Thus you have two instances of struct block_device [denote BDp] for
> > > > the partitions and one more for the whole disc [denote BDd]. When
> > > > you read from partition, you end up poking BDp, which pushes the
> > > > request up into BDd. This in turn calls USB-flashdisc-
> > > > block_controller_driver [call it UFc]. For flash disc to read data,
> > > > it needs to do some USB transfers. These are provided by USB host
> > > > controller [UHC]. Thus you need some glue between UHC and UFc ...
> > > > this is what I'm talking about.
> > > 
> > > there should be no "UFc", your "BDd" driver should talk directly to
> > > your "UHC"
> > 
> > So my generic partition implementation (BDd) would have to implement USB
> > flashdisc stuff, correct? This makes no sense.
> 
> no. your generic USB flash would have to implement USB flashdisc stuff,
> your generic partition implements block_device operations on top of other
> block_device (aka diosk, memory card, USB flash)

Ok, so in your parlance, the block_device is either "partition/disc" or a "SD 
card controller driver" or "USB flashdisc driver" ? You are mixing these two 
things together?

> please read the letters you came up with right. (maybe after getting some
> sleep by the looks of it)

I'd prefer to read some documented code.

> the point you are not getting is that there should be more block_device
> drivers than there is now - one for partitions, one for disk, one for USB
> flash, one for SD and so on, each one using a different parent API

Ok, now I understand your intention. Split it -- make partitions separate, since 
this is flat out confusing!

Make partitions / whole disc a separate thing ...
Make USB flash driver / SD card driver / etc. another thing ...

You can not mix these two together, it makes no sense.

> > > (a driver that has blockdev API on one end, USB on the other)
> > 
> > Ok, so how would this work, every partition implementation implements
> > upcalls for all USB, SCSI, SATA, IDE, SD, ... and gazilion other types of
> > drive it can sit on?
> 
> no, partition only implements call onto another block device
> 
> > > > Ok, I see the issue at hand. In case of a "regular drive", this
> > > > implements the IO directly. In case of SD, this is a proxy object
> > > > which interfaces with some SD-library and prepares the SD commands
> > > > and then pushes that up into the controller to do the job? Same
> > > > thing for USB flashes ?
> > > 
> > > not every block device will have a block controller as a parent (or
> > > parent-of- parent in case of a partition). there would be a
> > > blockdev-usb that has a USB hub as a parent, and a blockdev-mmc, that
> > > has a mmc/sdio controller as a parent.
> > 
> > So you would have a specific partition implementation for SD, SATA, IDE,
> > SCSI, USB ... ? This is flawed.
> 
> no, read above
> 
> > The partition should be a generic "thing" which knows nothing about where
> > it's sitting at. So is the whole drive, same thing, it just has
> > partitions hooked under it.
> > 
> > I'd expect a "block_controller" to be the proxy object under which the
> > block_device representing the disc is connected. And this
> > "block_controller" to be proxifying the requests to the respective
> > drivers (be it SD, SATA, whatever).
> 
> your idea is wrong - you expect there will always be only one block_device
> representig a "disk", and all the proxy would be done by the
> block_controller above it. this is not true

Any amount of "block_device" can be connected under the "block_controller". 
Given that "block_device" is a partition/disc _only_ and "block_controller" is 
the interface driver ... which is probably not true, so you lost me again.

I stop here, this discussion leads nowhere. Can you please write proper 
documentation from which I can get an idea how this exactly works? Ideally with 
diagrams ... doc/driver-model/UDM-block.txt would be a good place.

> 
> Pavel Herrmann

Best regards,
Marek Vasut