[RFC PATCH] phy: phy-mtk-tphy: Make USB PHY work on MT6735

[Chunfeng Yun]

On Mon, 2022-08-15 at 22:58 +0100, Yassine Oudjana wrote:
> The USB PHY on MT6735 seems to be a TPHY V1, but the FMREG base
> here is 0xf00 rather than 0x100. Since it is definitely not a V2
> or later as it has shared registers, would this mean that it is
> an even earlier revision, or one between V1 and V2? Or is the
> address used currently in this driver wrong?
> 
> Furthermore, there is one additional step needed in the power on
> sequence to make USB work on MT6735, which is to set the 10th bit
> of U3P_U2PHYDTM1. This was found through trial and error by adding
> the power on sequence from the downstream driver then stripping
> parts of it until a critical part is found. This specific part[1]
> was found under a comment saying "force enter device mode", but
> there are other bits that are cleared/set under the same comment,
> some of which are set in the same write as this bit, so it is
> unclear whether this bit is the one (or the only one) responsible
> for "forcing enter device mode". There is no documentation on this
> register (or any port registers for that matter) so this bit's
> name and function are unknown. Does anyone have more information
> on this? And if no information were to be found, would leaving
> this as a magic value, or with the placeholder name currently used
> (P2C_FORCE_ENTER_DEVICE_MODE) be acceptable?

Can't use force mode, there is side-effect with dual-role controller.

> 
> [1] 
> https://urldefense.com/v3/__https://gitlab.com/Tooniis/linux-samsung-grandpplte/-/blob/master/drivers/misc/mediatek/usb20/mt6735/usb20_phy.c*L404__;Iw!!CTRNKA9wMg0ARbw!26SfFWZy9e7D178mgOIWZ6iv4hM4OCTq4cndOspiaTxIQgvATHYlMsPCd4YiWq3S3Fal$
>  
> ---
>  drivers/phy/mediatek/phy-mtk-tphy.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/phy/mediatek/phy-mtk-tphy.c
> b/drivers/phy/mediatek/phy-mtk-tphy.c
> index 8ee7682b8e93..f54b8a9872bf 100644
> --- a/drivers/phy/mediatek/phy-mtk-tphy.c
> +++ b/drivers/phy/mediatek/phy-mtk-tphy.c
> @@ -23,7 +23,7 @@
>  /* version V1 sub-banks offset base address */
>  /* banks shared by multiple phys */
>  #define SSUSB_SIFSLV_V1_SPLLC		0x000	/* shared by
> u3 phys */
> -#define SSUSB_SIFSLV_V1_U2FREQ		0x100	/* shared by
> u2 phys */
> +#define SSUSB_SIFSLV_V1_U2FREQ		0xf00	/* shared by
> u2 phys */
>  #define SSUSB_SIFSLV_V1_CHIP		0x300	/* shared by u3 phys
> */
>  /* u2 phy bank */
>  #define SSUSB_SIFSLV_V1_U2PHY_COM	0x000
> @@ -119,6 +119,7 @@
>  
>  #define U3P_U2PHYDTM1		0x06C
>  #define P2C_RG_UART_EN			BIT(16)
> +#define P2C_FORCE_ENTER_DEVICE_MODE	BIT(10)
>  #define P2C_FORCE_IDDIG		BIT(9)
>  #define P2C_RG_VBUSVALID		BIT(5)
>  #define P2C_RG_SESSEND			BIT(4)
> @@ -579,6 +580,8 @@ static void u2_phy_instance_power_on(struct
> mtk_tphy *tphy,
>  		mtk_phy_set_bits(com + U3P_U2PHYDTM0, P2C_RG_SUSPENDM |
> P2C_FORCE_SUSPENDM);
>  	}
>  	dev_dbg(tphy->dev, "%s(%d)\n", __func__, index);
> +
> +	mtk_phy_set_bits(com + U3P_U2PHYDTM1,
> P2C_FORCE_ENTER_DEVICE_MODE);
>  }
>  
>  static void u2_phy_instance_power_off(struct mtk_tphy *tphy,