Re: [PATCH v4 4/6] mtd: rawnand: add NVIDIA Tegra NAND Flash controller driver

[Boris Brezillon]

On Tue, 12 Jun 2018 22:20:58 +0200
Stefan Agner <stefan@xxxxxxxx> wrote:

> On 12.06.2018 17:24, Jens Axboe wrote:
> > On 6/12/18 3:17 AM, Stefan Agner wrote:  
> >> [also added Jens Axboe]
> >>
> >> On 12.06.2018 10:27, Boris Brezillon wrote:  
> >>> On Tue, 12 Jun 2018 10:06:42 +0200
> >>> Stefan Agner <stefan@xxxxxxxx> wrote:
> >>>  
> >>>> On 12.06.2018 02:03, Dmitry Osipenko wrote:  
> >>>>> On Monday, 11 June 2018 23:52:22 MSK Stefan Agner wrote:  
> >>>>>> Add support for the NAND flash controller found on NVIDIA
> >>>>>> Tegra 2 SoCs. This implementation does not make use of the
> >>>>>> command queue feature. Regular operations/data transfers are
> >>>>>> done in PIO mode. Page read/writes with hardware ECC make
> >>>>>> use of the DMA for data transfer.
> >>>>>>
> >>>>>> Signed-off-by: Lucas Stach <dev@xxxxxxxxxx>
> >>>>>> Signed-off-by: Stefan Agner <stefan@xxxxxxxx>
> >>>>>> ---
> >>>>>>  MAINTAINERS                       |    7 +
> >>>>>>  drivers/mtd/nand/raw/Kconfig      |    6 +
> >>>>>>  drivers/mtd/nand/raw/Makefile     |    1 +
> >>>>>>  drivers/mtd/nand/raw/tegra_nand.c | 1248 +++++++++++++++++++++++++++++
> >>>>>>  4 files changed, 1262 insertions(+)
> >>>>>>  create mode 100644 drivers/mtd/nand/raw/tegra_nand.c
> >>>>>>  
> >>>> [snip]  
> >>>>>> +static int tegra_nand_cmd(struct nand_chip *chip,
> >>>>>> +			 const struct nand_subop *subop)
> >>>>>> +{
> >>>>>> +	const struct nand_op_instr *instr;
> >>>>>> +	const struct nand_op_instr *instr_data_in = NULL;
> >>>>>> +	struct tegra_nand_controller *ctrl = to_tegra_ctrl(chip->controller);
> >>>>>> +	unsigned int op_id, size = 0, offset = 0;
> >>>>>> +	bool first_cmd = true;
> >>>>>> +	u32 reg, cmd = 0;
> >>>>>> +	int ret;
> >>>>>> +
> >>>>>> +	for (op_id = 0; op_id < subop->ninstrs; op_id++) {
> >>>>>> +		unsigned int naddrs, i;
> >>>>>> +		const u8 *addrs;
> >>>>>> +		u32 addr1 = 0, addr2 = 0;
> >>>>>> +
> >>>>>> +		instr = &subop->instrs[op_id];
> >>>>>> +
> >>>>>> +		switch (instr->type) {
> >>>>>> +		case NAND_OP_CMD_INSTR:
> >>>>>> +			if (first_cmd) {
> >>>>>> +				cmd |= COMMAND_CLE;
> >>>>>> +				writel_relaxed(instr->ctx.cmd.opcode,
> >>>>>> +					       ctrl->regs + CMD_REG1);
> >>>>>> +			} else {
> >>>>>> +				cmd |= COMMAND_SEC_CMD;
> >>>>>> +				writel_relaxed(instr->ctx.cmd.opcode,
> >>>>>> +					       ctrl->regs + CMD_REG2);
> >>>>>> +			}
> >>>>>> +			first_cmd = false;
> >>>>>> +			break;
> >>>>>> +		case NAND_OP_ADDR_INSTR:
> >>>>>> +			offset = nand_subop_get_addr_start_off(subop, op_id);
> >>>>>> +			naddrs = nand_subop_get_num_addr_cyc(subop, op_id);
> >>>>>> +			addrs = &instr->ctx.addr.addrs[offset];
> >>>>>> +
> >>>>>> +			cmd |= COMMAND_ALE | COMMAND_ALE_SIZE(naddrs);
> >>>>>> +			for (i = 0; i < min_t(unsigned int, 4, naddrs); i++)
> >>>>>> +				addr1 |= *addrs++ << (BITS_PER_BYTE * i);
> >>>>>> +			naddrs -= i;
> >>>>>> +			for (i = 0; i < min_t(unsigned int, 4, naddrs); i++)
> >>>>>> +				addr2 |= *addrs++ << (BITS_PER_BYTE * i);
> >>>>>> +			writel_relaxed(addr1, ctrl->regs + ADDR_REG1);
> >>>>>> +			writel_relaxed(addr2, ctrl->regs + ADDR_REG2);
> >>>>>> +			break;
> >>>>>> +
> >>>>>> +		case NAND_OP_DATA_IN_INSTR:
> >>>>>> +			size = nand_subop_get_data_len(subop, op_id);
> >>>>>> +			offset = nand_subop_get_data_start_off(subop, op_id);
> >>>>>> +
> >>>>>> +			cmd |= COMMAND_TRANS_SIZE(size) | COMMAND_PIO |
> >>>>>> +				COMMAND_RX | COMMAND_A_VALID;
> >>>>>> +
> >>>>>> +			instr_data_in = instr;
> >>>>>> +			break;
> >>>>>> +
> >>>>>> +		case NAND_OP_DATA_OUT_INSTR:
> >>>>>> +			size = nand_subop_get_data_len(subop, op_id);
> >>>>>> +			offset = nand_subop_get_data_start_off(subop, op_id);
> >>>>>> +
> >>>>>> +			cmd |= COMMAND_TRANS_SIZE(size) | COMMAND_PIO |
> >>>>>> +				COMMAND_TX | COMMAND_A_VALID;
> >>>>>> +
> >>>>>> +			memcpy(&reg, instr->ctx.data.buf.out + offset, size);
> >>>>>> +			writel_relaxed(reg, ctrl->regs + RESP);
> >>>>>> +
> >>>>>> +			break;
> >>>>>> +		case NAND_OP_WAITRDY_INSTR:
> >>>>>> +			cmd |= COMMAND_RBSY_CHK;
> >>>>>> +			break;
> >>>>>> +
> >>>>>> +		}
> >>>>>> +	}
> >>>>>> +
> >>>>>> +	cmd |= COMMAND_GO | COMMAND_CE(ctrl->cur_cs);
> >>>>>> +	writel_relaxed(cmd, ctrl->regs + COMMAND);
> >>>>>> +	ret = wait_for_completion_io_timeout(&ctrl->command_complete,
> >>>>>> +					     msecs_to_jiffies(500));  
> >>>>>
> >>>>> It's not obvious to me whether _io_ variant is appropriate to use here, would
> >>>>> be nice if somebody could clarify that. Maybe block/ already does the IO
> >>>>> accounting itself and hence the IO time would be counted twice in that case.  
> >>>>
> >>>> Good that you bring this up.
> >>>>
> >>>> I don't think that there is any higher layer which could take care of
> >>>> accounting. Usually, with raw nand there is no block layer involved
> >>>> anyway.
> >>>>
> >>>> In a quick test it seems that only when using wait_for_completion_io I/O
> >>>> is properly accounted in the "wait" section of top.
> >>>>
> >>>> So far only a single driver (omap2) used the _io variant, but I think it
> >>>> is the right thing to do! After all, it is I/O...
> >>>>
> >>>> Boris or any other MTD maintainer, any comment on this?  
> >>>
> >>> Given this definition of io_schedule_timeout() [1] (which is used when
> >>> you call wait_for_completion_io_timeout()), I'd say it's not useful to
> >>> use the _io_ version, simply because MTD devs are not exposed as blk
> >>> devices, and thus don't need the blk_schedule_flush_plug() that is done
> >>> is io_schedule_prepare(). But that also means MTD I/Os are not
> >>> accounted as I/Os :-(.  
> >>
> >> Documentation of wait_for_completion_io says:
> >> "The caller is accounted as waiting for IO (which traditionally means
> >> blkio only)."
> >>
> >> Which sounds as if it using _io is only an accounting thing...  
> > 
> > Yes, you should only use it for waiting for IO off a system call
> > read path. So block IO, or file system IO. Don't use it for internal
> > IO that isn't related to that.  
> 
> I guess that would be the case here, since MTD page read/writes are
> typically file system IOs (e.g. UBIFS).
> 
> The problem is just that is not block related at all since it uses the
> MTD subsystem... And it seems that the _io variants besides accounting,
> also take a role in the block subsystems device plugging mechanism. What
> is unclear to me if using the _io variant from the MTD subsystem
> potentially disturbs the plugging mechanism...

How about doing that in 2 steps: first use the non-io version as other
drivers do, and, depending on how this discussion evolves, switch to
the _io_ version if it appears to be the right thing to do.