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//! Support for ACPI RSDP (Root System Descriptor Pointer).

#![no_std]


use core::mem;
use memory::{
    allocate_frames_by_bytes_at, allocate_pages_by_bytes, BorrowedMappedPages, MappedPages,
    PageTable, PhysicalAddress, PteFlags,
};
use zerocopy::FromBytes;

/// The starting physical address of the region of memory where the RSDP table exists.
const RSDP_SEARCH_START: usize = 0xE_0000;
/// The ending physical address of the region of memory where the RSDP table exists.
const RSDP_SEARCH_END:   usize = 0xF_FFFF;
/// The byte-string signature of the RSDP in memory.
const RSDP_SIGNATURE: &[u8; 8] = b"RSD PTR ";
/// The RSDP signature is always aligned on a 16-byte boundary.
const RSDP_SIGNATURE_ALIGNMENT: usize = 16;

/// The Root System Descriptor Pointer,
/// which contains the address of the RSDT (or XSDT),
/// among other items.  
#[derive(Copy, Clone, Debug, FromBytes)]
#[repr(packed)]
#[allow(dead_code)]
pub struct Rsdp {
    signature: [u8; 8],
    checksum: u8,
    oemid: [u8; 6],
    revision: u8,
    rsdt_address: u32,
    length: u32,
    xsdt_address: u64,
    extended_checksum: u8,
    reserved: [u8; 3],
}
const _: () = assert!(core::mem::size_of::<Rsdp>() == 36);
const _: () = assert!(core::mem::align_of::<Rsdp>() == 1);

impl Rsdp {
    /// Search for the RSDP in the BIOS memory area from 0xE_0000 to 0xF_FFFF.
    /// Returns the RSDP structure and the pages that are currently mapping it.
    pub fn get_rsdp(page_table: &mut PageTable) -> Result<BorrowedMappedPages<Rsdp>, &'static str> {
        let size: usize = RSDP_SEARCH_END - RSDP_SEARCH_START;
        let pages = allocate_pages_by_bytes(size).ok_or("couldn't allocate pages")?;
        let frames_to_search = allocate_frames_by_bytes_at(PhysicalAddress::new_canonical(RSDP_SEARCH_START), size)
            .map_err(|_e| "Couldn't allocate physical frames when searching for RSDP")?;
        let mapped_pages = page_table.map_allocated_pages_to(pages, frames_to_search, PteFlags::new().valid(true))?;
        Rsdp::search(mapped_pages)
    }

    /// Searches a region of memory for the RSDP, which is identified by the "RSD PTR " signature.
    fn search(region: MappedPages) -> Result<BorrowedMappedPages<Rsdp>, &'static str> {
        let size = region.size_in_bytes() - mem::size_of::<Rsdp>();
        let signature_length = mem::size_of_val(RSDP_SIGNATURE);
        let mut found_offset: Option<usize> = None;

        {
            let region_slice: &[u8] = region.as_slice(0, size)?;
            for offset in (0..size).step_by(RSDP_SIGNATURE_ALIGNMENT) {
                if &region_slice[offset..(offset + signature_length)] == RSDP_SIGNATURE {
                    found_offset = Some(offset);
                }
            }
        }

        found_offset
            .ok_or("couldn't find RSDP signature in BIOS memory")
            .and_then(|offset| region.into_borrowed(offset)
                .map_err(|(_mp, err)| err)
            )
    }

    pub fn from_address(
        address: PhysicalAddress,
        page_table: &mut PageTable,
    ) -> Result<BorrowedMappedPages<Rsdp>, &'static str> {
        let size = mem::size_of::<Rsdp>();
        let pages = allocate_pages_by_bytes(size).ok_or("couldn't allocate pages")?;
        let frames = allocate_frames_by_bytes_at(address, size)
            .map_err(|_e| "couldn't allocate physical frames for RSDP")?;
        let mapped_pages = page_table.map_allocated_pages_to(pages, frames, PteFlags::new().valid(true))?;
        mapped_pages.into_borrowed(address.frame_offset()).map_err(|(_, e)| e)
    }

    /// Returns the `PhysicalAddress` of the RSDT or XSDT.
    pub fn sdt_address(&self) -> PhysicalAddress {
        if self.revision >= 2 {
            PhysicalAddress::new_canonical(self.xsdt_address as usize)
        } else {
            PhysicalAddress::new_canonical(self.rsdt_address as usize)
        }
    }
}