Re: [PATCH 1/3] dt-bindings: mtd: stm32_fmc2: add STM32 FMC2 NAND controller documentation

[Boris Brezillon <boris.brezillon@bootlin.com>]

Hi Christophe,

On Mon, 24 Sep 2018 18:36:27 +0200
Christophe Kerello <christophe.kerello@st.com> wrote:

> >> +- st,fmc2_timings: array of 8 bytes for NAND timings. The meanings of
> >> +  these bytes are:
> >> +  byte 0 TCLR      : CLE to RE delay in number of AHB clock cycles, only 4 bits
> >> +                     are valid. Zero means one clock cycle, 15 means 16 clock
> >> +                     cycles.
> >> +  byte 1 TAR       : ALE to RE delay, 4 bits are valid. Same format as TCLR.
> >> +  byte 2 THIZ      : number of HCLK clock cycles during which the data bus is
> >> +                     kept in Hi-Z (tristate) after the start of a write access.
> >> +                     Only valid for write transactions. Zero means 1 cycle,
> >> +                     255 means 256 cycles.
> >> +  byte 3 TWAIT     : number of HCLK clock cycles to assert the command to the
> >> +                     NAND flash in response to SMWAITn. Zero means 1 cycle,
> >> +                     255 means 256 cycles.
> >> +  byte 4 THOLD_MEM : common memory space timing
> >> +                     number of HCLK clock cycles to hold the address (and data
> >> +                     when writing) after the command deassertion. Zero means
> >> +                     1 cycle, 255 means 256 cycles.
> >> +  byte 5 TSET_MEM  : common memory space timing
> >> +                     number of HCLK clock cycles to assert the address before
> >> +                     the command is asserted. Zero means 1 cycle, 255 means 256
> >> +                     cycles.
> >> +  byte 6 THOLD_ATT : attribute memory space timing
> >> +                     number of HCLK clock cycles to hold the address (and data
> >> +                     when writing) after the command deassertion. Zero means
> >> +                     1 cycle, 255 means 256 cycles.
> >> +  byte 7 TSET_ATT  : attribute memory space timing
> >> +                     number of HCLK clock cycles to assert the address before
> >> +                     the command is asserted. Zero means 1 cycle, 255 means 256
> >> +                     cycles.  
> > 
> > Let me review the driver but this array of timings is really
> > suspicious. I am pretty sure you don't need it in the DT.  
> 
> "st,fmc2-timings" is an optional property that allow the end user to 
> overwrite the timings calculated by setup_data_interface callback. By 
> setting this property in the NAND flash memory device tree node, the end 
> user could have a better throughput. For NON ONFI SLC NAND, timing mode 
> 0 is often used.

Exactly the kind of tweaking I'd like to avoid. If the NAND is not ONFI,
the vendor driver (nand_<manufacturer>.c) can overwrite
chip->default_onfi_timing_mode, and if the ONFI timings modes are not
exactly matching the NAND spec and you need the exact timings, then we
should consider adding a manufacturer hook to let the manufacturer
driver tweak the timings. In any case, I'm not willing to accept
timings description in the DT.

Regards,

Boris