Re: [PATCH v4 2/7] drm/tinydrm: Add helper functions

[Noralf TrÃnnes]

Den 12.02.2017 12.50, skrev Andy Shevchenko:
> On Sat, Feb 11, 2017 at 8:48 PM, Noralf TrÃnnes <noralf@xxxxxxxxxxx> wrote:
> > Add common functionality needed by many tinydrm drivers.
> > +int tinydrm_enable_backlight(struct backlight_device *backlight)
> > +{
> > +       unsigned int old_state;
> > +       int ret;
> > +
> > +       if (!backlight)
> > +               return 0;
> > +
> > +       old_state = backlight->props.state;
> > +       backlight->props.state &= ~BL_CORE_FBBLANK;
> > +       DRM_DEBUG_KMS("Backlight state: 0x%x -> 0x%x\n", old_state,
> > +                     backlight->props.state);
> "%#x" ?
> (And elsewhere)
>
> > +#if IS_ENABLED(CONFIG_SPI)
> > +size_t tinydrm_spi_max_transfer_size(struct spi_device *spi, size_t max_len)
> > +{
> > +       size_t ret;
> > +
> > +       ret = min(spi_max_transfer_size(spi), spi->master->max_dma_len);
> I don't get why DMA constrain somehow affects this framework?

The reason is that spi-bcm2835 reverts to PIO on larger buffers.
Looking at __spi_map_msg() and spi_map_buf() it becomes clear that the
core breaks up the buffer into manageable parts. So this must be a bug
in spi-bcm2835 (and spi-pxa2xx) since no other drivers have a upper
limit in the .can_dma() callback.
So you're rightly confused, with drivers fixed, this limit can be lifted.

> What if max_dma_len is zero (imagine SPI master that works only by PIO
> by some reason)?

It can't be zero:

int spi_register_master(struct spi_master *master)
{
...
    if (!master->max_dma_len)
        master->max_dma_len = INT_MAX;

> > +       if (max_len)
> > +               ret = min(ret, max_len);
> > +       if (spi_max)
> > +               ret = min_t(size_t, ret, spi_max);
> > +       ret &= ~0x3;
> Why alignment is that? Why do we need it? Isn't a busyness of SPI
> framework to cope with it?

This minimum capping doesn't look good. I should probably put >16
instead, that would cover the minimum for the 9-bit emulation code.

The reason I let the user change the transfer size, is that some usb
audio card on Raspberry Pi stuttered with 4k fbtft transfers.

> > +       if (ret < 4)
> It's effectively check for 0.
>
> > +               ret = 4;
> > +
> > +       return ret;
> > +}
> > +EXPORT_SYMBOL(tinydrm_spi_max_transfer_size);
> > +static void
> > +tinydrm_dbg_spi_print(struct spi_device *spi, struct spi_transfer *tr,
> > +                     const void *buf, int idx, bool tx)
> > +{
> > +       u32 speed_hz = tr->speed_hz ? tr->speed_hz : spi->max_speed_hz;
> > +       char linebuf[3 * 32];
> > +
> > +       hex_dump_to_buffer(buf, tr->len, 16,
> > +                          DIV_ROUND_UP(tr->bits_per_word, 8),
> > +                          linebuf, sizeof(linebuf), false);
> > +
> > +       printk(KERN_DEBUG
> > +              "    tr(%i): speed=%u%s, bpw=%i, len=%u, %s_buf=[%s%s]\n", idx,
> > +              speed_hz > 1000000 ? speed_hz / 1000000 : speed_hz / 1000,
> > +              speed_hz > 1000000 ? "MHz" : "kHz", tr->bits_per_word, tr->len,
> > +              tx ? "tx" : "rx", linebuf, tr->len > 16 ? " ..." : "");
> I hope at some point we will have some extension to print speeds,
> sizes and so on based on algo in string_get_size().
>
> > +}
> > +int tinydrm_spi_transfer(struct spi_device *spi, u32 speed_hz,
> > +                        struct spi_transfer *header, u8 bpw, const void *buf,
> > +                        size_t len)
> > +{
> > +       struct spi_transfer tr = {
> > +               .bits_per_word = bpw,
> > +               .speed_hz = speed_hz,
> > +       };
> > +       struct spi_message m;
> > +       u16 *swap_buf = NULL;
> > +       size_t max_chunk;
> > +       size_t chunk;
> > +       int ret = 0;
> > +
> > +       if (WARN_ON_ONCE(bpw != 8 && bpw != 16))
> > +               return -EINVAL;
> > +
> > +       max_chunk = tinydrm_spi_max_transfer_size(spi, 0);
> > +
> > +       if (drm_debug & DRM_UT_DRIVER)
> > +               pr_debug("[drm:%s] bpw=%u, max_chunk=%zu, transfers:\n",
> > +                        __func__, bpw, max_chunk);
> For all of your dev_dbg() / pr_debug() __func__ argument might be
> redundant. Dynamic Debug may include this by request from user.
>
> > +/**
> > + * tinydrm_machine_little_endian - Machine is little endian
> > + *
> > + * Returns:
> > + * true if *defined(__LITTLE_ENDIAN)*, false otherwise
> > + */
> > +static inline bool tinydrm_machine_little_endian(void)
> > +{
> > +#if defined(__LITTLE_ENDIAN)
> > +       return true;
> > +#else
> > +       return false;
> > +#endif
> > +}
> Hmm... What is the typical code of a caller for this?

If the bus can't do 16-bit natively, it will have to swap the bytes on
little endian machines before transfer as 8-bit (Raspberry Pi can't do
16-bit SPI with it's DMA capable controller).
So this function is to avoid #ifdef's elsewhere.