Re: [PATCH v3 4/9] of: mtd: add documentation for the ONFI NAND timing mode property

[Brian Norris]

On Tue, May 20, 2014 at 09:30:33PM +0200, Boris BREZILLON wrote:
> Hi Brian,
> 
> On 20/05/2014 20:25, Brian Norris wrote:
> > Hi Boris,
> >
> > On Wed, Mar 12, 2014 at 07:07:39PM +0100, Boris BREZILLON wrote:
> >> Add documentation for the ONFI NAND timing mode property.
> >>
> >> Signed-off-by: Boris BREZILLON <b.brezillon.dev@xxxxxxxxx>
> >> ---
> >>  Documentation/devicetree/bindings/mtd/nand.txt |    8 ++++++++
> >>  1 file changed, 8 insertions(+)
> >>
> >> diff --git a/Documentation/devicetree/bindings/mtd/nand.txt b/Documentation/devicetree/bindings/mtd/nand.txt
> >> index b53f92e..2046027 100644
> >> --- a/Documentation/devicetree/bindings/mtd/nand.txt
> >> +++ b/Documentation/devicetree/bindings/mtd/nand.txt
> >> @@ -19,3 +19,11 @@ errors per {size} bytes".
> >>  The interpretation of these parameters is implementation-defined, so not all
> >>  implementations must support all possible combinations. However, implementations
> >>  are encouraged to further specify the value(s) they support.
> >> +
> >> +- onfi,nand-timing-mode: an integer encoding the supported ONFI timing modes of
> >> +  the NAND chip. Each supported mode is represented as a bit position (i.e. :
> >> +  mode 0 and 1 => (1 << 0) | (1 << 1) = 0x3).
> >> +  This is only used when the chip does not support the ONFI standard.
> >> +  The last bit set represent the closest mode fulfilling the NAND chip timings.
> >> +  For a full description of the different timing modes see this document:
> >> +  www.onfi.org/~/media/ONFI/specs/onfi_3_1_spec.pdf
> > I'm not 100% convinced this property should go in the device tree. With
> > most other flash properties (device size, page size, and even minimum
> > ECC requirements), we try to auto-detect these parameters to some
> > extent. ONFI makes it easy for some class of chips, but for others, we
> > typically rely on an in-kernel device ID table or ID decoding heuristic
> > -- we don't require a DT description of every property of the flash. So
> > what makes this property different?
> 
> AFAICT nothing, but the same goes for the ECC requirements, and we've
> recently added DT bindings to define these requirements.
> I'm not telling we should drop these ECC requirements bindings (actually
> I'm using them :-)), but what's different with the timings requirements ?

ECC selection is not quite as scientific; with ECC, there are external
factors that influence the ECC mode that you should use, since any data
read/written from Linux has to be compatible with any data read/written
with another entity (e.g., bootloader). Note that the ECC bindings do
not represent a property of the flash chip itself (i.e., they don't hold
the "minimum required ECC strength"), but of the entire flash system
(i.e., "what ECC must I use to play nicely with the rest of the world").

With timing modes, this is purely a property of the flash chip, and we
do not have to synchronize it with the bootloader. We don't exactly care
if a bootloader and Linux use slightly different timing modes.

> Moreover, we will end up with a lot of new entries in the device ID
> table if we decide to put these informations in this table.

Yes, that could be a problem.

What sort of non-ONFI flash chips do you have that need this property?
And what timing mode(s) do they use? Is there, for instance, a pattern
such that all Hynix MLC of a certain generation use a particular timing
mode?

> > I realize that we may not include device ID entries for every flash that
> > you need in the ID table (although we still are able to detect the
> > important properties accurately, like page and block size). But would it
> > suffice to default these flash to a lowest common timing mode, like mode
> > 0?
> 
> IMHO this is not a good solution: you'll end up with lower perfomances
> on most of the supported NAND chips and I'm not sure this is what we want.

No, we wouldn't want to always use mode 0. But it's possible we can get
good enough heuristics for most flash, if we can integrate timing modes
into the current extended ID decoding. Not sure.

I'm also concerned here that this kind of binding will be difficult to
use properly. A user/developer/board-designer would have to read the
datasheet and compare all its values to the ONFI spec to find the
closest match, and they would have to do this for each new flash they
use. If we can help them by autodetecting this, that would be great.

> > If no other option works well, then I am still open to describing the
> > supported timing modes in the DT.
> >
> > BTW, this bitfield property looks kinda strange to me. Do non-ONFI flash
> > typically support multiple timing modes? And if so, how are we supposed
> > to *change* modes? AFAIK, ONFI provides the only standard for
> > configuring the flash's timing mode. So maybe you're really only wanting
> > a "default timing mode" property that is a single integer, not a
> > bitfield.
> 
> Indeed, I based it on the ONFI NAND timings mode model, but AFAIK (tell
> me if I'm wrong), it should work because most of the timings are min
> requirements.
> This means, even if you provide slower signals transitions, the NAND
> will work as expected.

So you're saying that even though the chip actually specifies a single
set of timings, you would describe this as a bitmask of several
supported ONFI timing modes, up to the "max performance"?

Is there ever a case where (for instance) a non-ONFI flash supports the
equivalent of timing mode 3, but it does not support mode 2 or 1?

> But I can modify the bindings to just encode the maximum supported
> timing mode.

AIUI, the non-ONFI datasheets really only specify a single timing mode,
so I think we should only specify the "max." And as a bonus, this
actually makes the binding easier to use. A driver does not care about
how many different modes are supported; it only needs to know what the
max is.

Brian
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