... > We're dealing with a similar issue in other drivers currently, and I > think it's worth straightening out the issue for all systems. > > On Wed, Nov 13, 2013 at 03:58:02PM +0400, Alexander Shiyan wrote: > > This patch adds a property to automatically determine the NAND > > bus width by CFI/ONFI information from chip. This property works > > if the bus width is not specified explicitly. > > This issue brings up a few questions in my mind, which are relevant to > device tree in general. > > First of all, do we need a device tree property at all, for something > that is auto-detectable? > > Related: is a device tree consumer (like Linux) supposed to be a > validator, or simply a best-effort? I'm considering the following case: > if Linux is allowed to auto-detect some property which also has a device > tree binding (e.g., "nand-bus-width", in > Documentation/devicetree/bindings/mtd/nand.txt), what should happen if > the binding happens to be incorrect? IOW, what if the device tree > specifies buswidth is x16, but Linux correctly detects it as x8? > Shouldn't we make the best effort to bring the hardware up, regardless > of what the device tree says? > > So for something like this GPIO driver, I'm thinking that Linux should > just use NAND_BUSWIDTH_AUTO all the time [*]. But (as I'm hinting above) > that would allow DTB firmware implementors to be lazier and have a > technically-incorrect "nand-bus-width" or "bank-width" binding, since > they know it can reliably be detected and overridden by Linux. > > [*] Except where resource_size(res) < 2, as Alexander already has in > this patch. > > > Signed-off-by: Alexander Shiyan <shc_work@xxxxxxx> > > --- > > .../devicetree/bindings/mtd/gpio-control-nand.txt | 3 +++ > > drivers/mtd/nand/gpio.c | 16 ++++++++++++---- > > 2 files changed, 15 insertions(+), 4 deletions(-) > > > > diff --git a/Documentation/devicetree/bindings/mtd/gpio-control-nand.txt b/Documentation/devicetree/bindings/mtd/gpio-control-nand.txt > > index 36ef07d..fe4e960 100644 > > --- a/Documentation/devicetree/bindings/mtd/gpio-control-nand.txt > > +++ b/Documentation/devicetree/bindings/mtd/gpio-control-nand.txt > > @@ -19,6 +19,9 @@ Optional properties: > > defaults to 1 byte. > > - chip-delay : chip dependent delay for transferring data from array to > > read registers (tR). If not present then a default of 20us is used. > > +- gpio-control-nand,bank-width-auto : Device bus width is determined > > + automatically by CFI/ONFI information from chip if "bank-width" > > + parameter is omitted (Boolean). > > If we do resort to a new binding for auto-buswidth, it should be a > generic one that all NAND drivers can use. Maybe a separate boolean > "nand-bus-width-auto" and if it is present, then it overrules the > presence (or lack) of the "nand-bus-width" boolean property? > Or is it possible to extend "nand-bus-width" to allow the value of 0 to > mean automatic? This was be my first implementation of this feature, but rejected... http://permalink.gmane.org/gmane.linux.drivers.mtd/47302 --- ��.n��������+%������w��{.n����z�{��ܨ}���Ơz�j:+v�����w����ޙ��&�)ߡ�a����z�ޗ���ݢj��w�f
