Re: [PATCH v6 2/2] mtd: rawnand: meson: add support for Amlogic NAND flash controller

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hello,

Boris Brezillon <boris.brezillon@bootlin.com> wrote on Tue, 6 Nov 2018
11:22:06 +0100:

> On Tue, 6 Nov 2018 18:00:37 +0800
> Liang Yang <liang.yang@amlogic.com> wrote:
> 
> > On 2018/11/6 17:28, Boris Brezillon wrote:  
> > > On Tue, 6 Nov 2018 17:08:00 +0800
> > > Liang Yang <liang.yang@amlogic.com> wrote:
> > >     
> > >> On 2018/11/5 23:53, Boris Brezillon wrote:    
> > >>> On Fri, 2 Nov 2018 00:42:21 +0800
> > >>> Jianxin Pan <jianxin.pan@amlogic.com> wrote:
> > >>>        
> > >>>> +
> > >>>> +static inline u8 meson_nfc_read_byte(struct mtd_info *mtd)
> > >>>> +{
> > >>>> +	struct nand_chip *nand = mtd_to_nand(mtd);
> > >>>> +	struct meson_nfc *nfc = nand_get_controller_data(nand);
> > >>>> +	u32 cmd;
> > >>>> +
> > >>>> +	cmd = nfc->param.chip_select | NFC_CMD_DRD | 0;
> > >>>> +	writel(cmd, nfc->reg_base + NFC_REG_CMD);
> > >>>> +
> > >>>> +	meson_nfc_drain_cmd(nfc);    
> > >>>
> > >>> You probably don't want to drain the FIFO every time you read a byte on
> > >>> the bus, and I guess the INPUT FIFO is at least as big as the CMD
> > >>> FIFO, right? If that's the case, you should queue as much DRD cmd as
> > >>> possible and only sync when the user explicitly requests it or when
> > >>> the INPUT/READ FIFO is full.
> > >>>        
> > >> Register 'NFC_REG_BUF' can holds only 4 bytes, also DRD sends only one
> > >> nand cycle to read one byte and covers the 1st byte every time reading.
> > >> i think nfc controller is faster than nand cycle, but really it is not
> > >> high efficiency when reading so many bytes once.
> > >> Or use dma command here like read_page and read_page_raw.    
> > > 
> > > Yep, that's also an alternative, though you'll have to make sure the
> > > buffer passed through the nand_op_inst is DMA-safe, and use a bounce
> > > buffer when that's not the case.
> > >     
> > ok, i will try dma here.  
> 
> We should probably expose the bounce buf handling as generic helpers at
> the rawnand level:
> 
> void *nand_op_get_dma_safe_input_buf(struct nand_op_instr *instr)
> {
> 	void *buf;
> 
> 	if (WARN_ON(instr->type != NAND_OP_DATA_IN_INSTR))
> 		return NULL;
> 
> 	if (virt_addr_valid(instr->data.in) &&
> 	    !object_is_on_stack(instr->data.buf.in))
> 		return instr->data.buf.in;
> 
> 	return kzalloc(instr->data.len, GFP_KERNEL);
> }
> 
> void nand_op_put_dma_safe_input_buf(struct nand_op_instr *instr,
> 				    void *buf)
> {
> 	if (WARN_ON(instr->type != NAND_OP_DATA_IN_INSTR) ||
> 	    WARN_ON(!buf))
> 		return;
> 
> 	if (buf == instr->data.buf.in)
> 		return;
> 
> 	memcpy(instr->data.buf.in, buf, instr->data.len);
> 	kfree(buf);
> }
> 
> const void *nand_op_get_dma_safe_output_buf(struct nand_op_instr *instr)
> {
> 	void *buf;
> 
> 	if (WARN_ON(instr->type != NAND_OP_DATA_OUT_INSTR))
> 		return NULL;
> 
> 	if (virt_addr_valid(instr->data.out) &&
> 	    !object_is_on_stack(instr->data.buf.out))
> 		return instr->data.buf.out;
> 
> 	return kmemdup(instr->data.buf.out, GFP_KERNEL);
> }
> 
> void nand_op_put_dma_safe_output_buf(struct nand_op_instr *instr,
> 				    void *buf)
> {
> 	if (WARN_ON(instr->type != NAND_OP_DATA_OUT_INSTR) ||
> 	    WARN_ON(!buf))
> 		return;
> 
> 	if (buf != instr->data.buf.out)
> 		kfree(buf);
> }

Not that I am against such function, but maybe they should come with
comments stating that there is no reliable way to find if a buffer is
DMA-able at runtime and these are just sanity checks (ie. required, but
probably not enough). This is my understanding of Wolfram's recent talk
at ELCE [1]. I suppose using the CONFIG_DMA_API_DEBUG option could help
more reliably to find such issues.

[1] https://www.youtube.com/watch?v=JDwaMClvV-s

Thanks,
Miquèl